I don't think this can be considered a bug in the MTA. GTA SA has a coded water cannons limit of 3.

https://github.com/gta-reversed/gta-reversed-modern/blob/1b37b015fbda7957ebbc36dbe8d5e4a90ebb6891/source/game_sa/WaterCannons.h#L8

I don't think this can be considered a bug in the MTA. GTA SA has a coded water cannons limit of 3.
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Well yeah, but by that logic for instance not being able to enter a vehicle as a passenger wouldn't be an issue either, because it's not in the singleplayer. In a singleplayer environment there is no need for more than a few water cannons to exist, but in the context of a multiplayer there is, so while it's not a bug, because it's working perfectly as intended when the code was written, it's a restriction MTA should try to lift if possible, because it will be perceived as an unintended side effect of the nature of turning a singleplayer game into a multiplayer one. So this issue definitely has a right to exist, at least from my perspective.

Yes, you are right, but a fundamental question should be asked here - Will increasing the limit not cause further, much bigger problems, such as crashes or bugs with effects? After all, for some reason the game developers introduced such a restriction. This needs to be checked carefully to be able to clearly answer the question of whether the MTA can increase the water cannon limit.