Giant Shark can't attack unless defending player controls an Island.
Whenever Giant Shark blocks or becomes blocked by a creature that has been dealt damage this turn, Giant Shark gets +2/+0 and gains trample until end of turn.
When you control no Islands, sacrifice Giant Shark.
10/4/2004 Giant Shark only gains its bonus if the blocking creature was damaged before blocking was declared. Damaging the creature later will not give the Shark the bonus.
10/4/2004 The bonus happens when a previously damaged creature becomes a blocker and the bonus is not removed if that creature is removed from the blocking situation by any means.
10/4/2004 If a creature enters a blocking situation with the Giant Shark due to being part of a banded group of attackers, the Shark will get the bonus if that creature is damaged at that time.
10/4/2004 Giant Shark gets the bonus once for each creature it blocks or is blocked by that meets the criteria.