-High Attack and Speed, and decent Special Attack.
-Learns some good moves by level up and TM.
-Definitely not meant to take hits or be bulky.
-Evolves earlier than Shellos.
-Can make use of the Rain with its Swift Swim ability.
-Has pretty good stats all around, minus Speed and Defense.
-Can take a lot of hits.
-Only learns Water, Ground, and Normal moves by level up, so you may need to give it some TMs.
-Only has one Weakness.
-Has one pretty good ability.
In-game, almost anything can work as long as you know how to use it. I'd probably just pick one based on what the rest of your team is. Use Floatzel if you need a fast Pokemon to deal a lot of damage, and use Gastrodon if you need a bulkier Pokemon on your team to take hits.
Hope I helped!