This seems sensible to me.
The fact that Biremes were enabled at town phase dates from rP12240 which introduced tech requirements for warships. It seems to me that biremes have no particularly strong reason to not be in village_phase, given the commit.

That being said, I wonder if they should be slightly nerfed in consequence, or not?

That being said, I wonder if they should be slightly nerfed in consequence, or not?

Maybe lower <DefaultArrowCount> or <MaxArrowCount>?

I'm not a fan of this unless some test games show it to be and improvement. Islands games become super simplistic. It's all about just doing biremes from the get go and the snowball effect will be massive. You can't do fishing ships because then you get outnumbered and lose. It also pretty much means that very rarely will games go to Phase 2 since players won't have the eco for it. Even if they decide to sacrifice all map control for Phase 2 and Triremes it's not worth it due to Triremes not being so much stronger than Biremes.
In other maps where water control is an extra it could be an improvement over the current Phase 2 only warships, by adding another early point of contention. Although at the same time it could be a detriment because the defending player will always be ready to defend, because the attacking player can't rush to Phase 2 and catch the enemy offguard.

@ValihrAnt, if you're really opposed to a patch, you could consider clicking “Add Action...” → “Request Changes”, to make it clear it should not be merged as is.
In its defence, the reduction in default arrow count makes biremes significantly weaker. Moreover, I think having a small but weak warship-transport in the village phase, a decent one in town phase, and a powerful one in city phase makes sense (tech progression).
On the other hand, not all civs have biremes.
Anyway, additional playtesting is always welcome, also for other patches.

We can reduce the strength of the bireme a little more, so that the snowball is not so significant, Increasing your cost is also interesting.

Triremes not being so much stronger than Biremes.

So I wrote a scenario and did a bit of testing:

• without this patch, one trireme loses to two biremes
• with this patch, one trireme defeats two biremes, but loses to three
• with this patch and a small health tweak (bireme 800→700, trireme 1400→1500), one trireme defeats three biremes

Increasing your cost is also interesting.

Increasing the bireme cost? But then trireme cost should be increased too.

Increasing your cost is also interesting.

Don't think cost should be increased.

with this patch, one trireme defeats two biremes, but loses to three

I think that's a good spot for now. I presume you did the testing with the ships ungarrisoned, in which case the arrow count difference is 200%. When the ships are fully garrisoned the difference is 18%. Keep in mind that going up to Phase 2 costs the same as 10 soldiers. I'd presume the stronger eco of remaining in Phase 1 would also allow to get the armor upgrade much more easily.
Basically I'm unsure how this would play out without some testing.

What I did was launch Atlas, generate a map with water, go to one end of the map, place two biremes of player two facing one trireme there, go to another part of the map, place three biremes of player three facing one trireme of player four, save the scenario, the open 0 A.D., launch the map, and observe the results. And yes, the ships are ungarrisoned.
I don't know how it will work out in larger numbers. Ships have <BuildingAI>, which means they shoot at all enemies within range, rather than concentrating their fire at one at a time.

the arrow count difference is 200%. When the ships are fully garrisoned the difference is 18%.

How did you arrive at those numbers? A bireme has a default arrow count of 2, which this patch lowers to 1, and a max arrow count of 10. A trireme has 3 and 13. That works out as 1:3 and 10:13 in an one-vs-one fight; in a two-vs-one, it's 2:3 at start; in three-vs-one, it's 3:3 at start.

Keep in mind that going up to Phase 2 costs the same as 10 soldiers.

Yes, I know, and a trireme costs the same as three basic infantry. I have no idea which strategy will be more successful in practice.

Basically I'm unsure how this would play out without some testing.

That's the case with practically all gameplay patches. One can look at a patch and judge whether it seems a good idea, verify the actual changes are correct and complete, and maybe do a bit of limited testing. However, one can only discover the effect of the patch, if any, on gameplay as whole if it's committed and feedback is received from people actually playing the development version. If something turns out to have unbalanced the game severely, then it can always be reverted.
For this patch speficially, my main concern is that not all civs have biremes, thus putting some at a disadvantage in the village phase. Nonetheless, I don't consider that a serious enough objection against having this patch in, therefore I've accepted it.

Some civilizations do not have bireme it does not seem like a problem with the patch, this makes players make better decisions about which civilization to choose on water maps, maybe brit/gauls not a better choose.

Some civilizations do not have bireme it does not seem like a problem with the patch, this makes players make better decisions about which civilization to choose on water maps, maybe brit/gauls not a better choose.

I very much dislike that some civs can have an automatic civ lose situation.
I did a test game with ffm (Britons) vs me (Carthaginians) on Islands. Obviously I was able to get a big mass of ships before he reached P2. While he probably could've fought back if I left my ships ungarrisoned there was no chance with just 2 being fully garrisoned and a bunch of empty ships around them soaking damage. Another little problem is that if I wanted to I could keep him from fighting back at all, that is by putting 3 biremes around a dock I can deny his warship from ever being released.
With this change Gauls and Britons will become nearly useless on water, while I think Iberians could be fine due to having the fireship. So, either biremes need to be nerfed further or the Celtic civilizations could receive some sort of early game buff on water.

Why don't we move the gaules / brit trireme to village phase? the original idea of ​​the brit / gaul trireme was to have more hit points and less attack. We put it in phase two and reduce the attack a little bit, I think it's fair.

Or maybe keep the bireme as is and introduce a new, much weaker (e.g. 400 health, 1 arrow, 10 garrison capacity) village-phase ship for all civs.

I don't think it is necessary to create new models. as gaul and brit are extremely poor in water maps, putting trireme early can be interesting. we can lower the attack a bit (original design idea), or even increase the cost of the gaul / brit / (iber too?) triremes in proportion to their largest hp.