For the fun of it I counted the death income from water provinces in the late game perpetuality, where I play Rlyeh. I have searched maybe 60-65 of those provinces with death at level 9. My total income from water provinces in death gems are 21 (this game was created during the 3.08 time I belive). If it is worth searching those water provinces ?
Cost to search: 60+*3=180+ gems
Total income
hard to estimate since I didn't start with max income, but approximatly - at turn 85 now)=60*21=1200+
Generally speaking, search for sites even underwater if you can.