Water Strider. The snail can move unnaturally fast when in water. 60 ft of swim speed as it zooms across the water. 

Charge. If the Snail moves in water at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, the target takes an extra 7 (2d6) damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked prone.

Ram. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.

Shell Defense. The flail snail withdraws into its shell, gaining a +4 bonus to AC until it emerges. It can emerge from its shell as a bonus action on its turn.