Tidal Warrior and Islandwalk
Yes, you can do this.  Islandwalk prevents a creature from being blocked as long as the defending player controls an Island1.  It doesn't matter whether that Island used to be a different kind of land - after using Tidal Warrior's ability, your opponent controls an Island1, and creatures with Islandwalk cannot be blocked.



1. Note that "controls an Island" means "controls a permanent that has the subtype 'Island.'"  The name of the card is irrelevant.