TOPIC:

I'm not a fan of waterwalking at all, but with only 10 ships allowed there's no way to build a good oil network without waterwalking. So my suggestion is to increase the ship number to some reasonable value, say 100, or 500 as on long goal servers.

Maybe increase it to 15 but not 20.

On quick goal servers I would like to see boats allowed too like a limit of 5 or something.

nothing in this game kills the servers more than boats, therefor you will never see the limit of boats increase to anything above 25.

For example on server 2 long goal temperate. someone decided to go full out boat empire. suddenly server 2 took all the cpu power itself. leaving nothing for the others and resulting in each server being unresponsive and time eventualy go down.

even the strongest of todays servers will struggle with big maps loaded with boats
(you may wonder why boats cause this much harm, i honestly have no answer to give

Boat path finding on openttd is just very poor, and it still eats cpu. I think its due to a boat being able to take any water-path he wants unlike a train, which is bound to a certain track.
Personally I'd encourage to have no ships at all, but that would make the oil rigs a bit useless indeed (unless waterwalking)