What happens if I have no cards in my hand and use Manaphy's "Deep Sea Swirl" attack?
You always have a hand, even it you have zero cards in it. So you shuffle your deck and draw 5 cards.
Source: Unleashed FAQ (2010-05-13), PUI Rules Team (2010-05-13)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS2_3