The attack text of "Group Swim" simply states that you may search your deck for a {W} Pokémon and put it into your hand after revealing the card to your opponent. As Floatzel GL Lv. X is a {W} Pokémon, it can be searched with this attack. If the attack said to search for a Basic {W} Pokémon, then you would not be able to search out Floatzel GL Lv. X, as Lv. X cards do not have any particular stage until they enter play (at which point they inherit the stage of the Pokémon beneath them).
By that understanding then, any Ball engine (or any other trainer, supporter, attack, etc.) that specifically states you must search for a Basic Pokémon would not be used to search out any Lv. X cards, as they are not considered Basic Pokémon while in the deck. However, the actual card Poké Ball (HGSS-95) simply states to search your deck for "a Pokémon", meaning you are able to search out Lv. X cards with its effect.