You don't have to worry about that, Bottomswell wil never use Waterball if there is only one target to choose from.
Mug
Physical Attack
Formula: Physical
Pwr: 1x Base
PAt%: Weapon At%
Tar: 1 Op/All Op
Elm: As Weapon
Attempts to steal an item from the targetted enemy
Goes through all items that the target has in turn
Success Formula is as follows:
LvlDiff = 40 + Attacker's Level - Target's Level
If Sneak Glove is equipped, LvlDiff is capped at 100 minimum
LvlFactor = [512 * LvlDiff / 100]
Base Chance = Item Chance * LvlFactor / 256
If (Rnd(0..63) <= Base Chance): Stop here, WIN ITEM!
In other words, It does the same as a regular attack.