Question: I'm having mega trouble killing them: I've tried bombs, shotguns, etc. but they're still a nuisance!

Are there any tips or tricks to taking them out?

Answer: 
Fire won't work vs. Floaters because they are almost always standing in water. 
Poison doesn't seem to work (even with Plaguebearer), probably because that is the Floater's base elemental dmg.
**Shock modded weapons** seem to work the best of the elemental damage mods, and can be applied with a pistol or rifle at a safe distance. A Tesla Bomb knife might be able to do the trick, but I haven't tried in the higher levels, so I'm unsure if it is automatic --> one good possibility.
**Impact mods/weapons** (Shocking Crowbar, Heavy Duty Kanabo, Left Hand of Glova, or the Homerun Baseball Bat (developer's mod)) can be thrown/used and knock him down and then you can stomp on his head. --> this is nearly automatic, as long as you can get him down, you can rush in and stomp.
**Explosives** (Sticky Bomb knife, Deo-bomb, & Grenade) - These do the trick nicely, you just need to be careful in tight spaces like the Sewers of Moresby so that you don't hurt yourself in the process. 
**Vehicles** care free and easy, you can either kill them or avoid them, your choice.

As for the ***best***, it would be situational;
 - If you're in a confined space, try the impact+stomp route.
 - If you are at mid-range and have room to move, use a grenade/sticky bomb or two.
 - If you've got line of sight and distance, try a rifle/pistol with a shock/pride mod and just keep plugging away at him.
 - If you've got access to a vehicle, then by all means run him down (or go around him).
The EASIEST possible way for killing the floaters is the homerun baseball bat that has near 100% crit attack everytime.