Something I've found by talking to people is that they tend to use the blaster in single player, even when other ammunition is plentiful. I found a way to solve this in a certain low-gravity water level I am making.

Force players to run through a section with a fire shield, which is needed to get past a certain obstacle at the end. There will be enemies blocking your way, but the shield makes it impossible to use your blaster. Funny I never thought of doing this before.

Water levels are so boring without gimmicks like this. I'm also tempted to make a section which is infested with rats, but something tells me that under-water rats is a bit too ridiculous, even for me.