Slugs can swim somewhat but not very good.
So when the slugs get into water they will try to abandoned the water before they possibly drown.
Although slugs prefer wet and damp environments they can drown if they get too much water in their body.
Slugs could die in water if the slugs don't get out of the water and then they could possibly drown in the water.
Slugs do like wet environments though so you're more likely to see slugs when it's raining outside or when it's pretty humid outside.
When the slugs get into a bunch of water they will abandoned those areas though so that they don't drown.
Slugs aren't really good at swimming so they would try to get out of the water before it reached a point where they couldn't get out.
Whenever I've seen slugs in water they always got out of the water before it got too deep.
If you want to kill slugs though you can pour salt on the slug and the salt will dehydrate the slug and it will shrivel up and die.
But they could also drown in water if they can't get out.