But if they spit it then it would be toxic or acid?
Animal, spitting toxines, is still called venomous.
But acid is not from that family. In fact, i know no animal or plant, using acid as weapon. Except lemons.
And for use poison as weapon against aquanaut, we need to penetrate his armor first. Well, if his suit is sealed. But acid can penetrate armor by self, though.
In regards to laser they are supposed to be 50% weaker under water . Would it be possible to make it so that laser deal more damage on land than under water?
It is possible for armor sets, very doubt about weapons. But why 50%? Water disperse light around 1000 times more than air. So i decided to divide laser weapons to surface-only and underwater-short-ranged.