The Island of Dread expansion does not have a blue adventure card deck as such, but does have a sea deck which is kept separate from the other adventure card decks. Does this mean:

A: Sea deck encounters are worth 3 experience (blue adventure counter) when defeated unless the reward gives a different amount.

or

B: Sea deck encounters give no experience unless stated in the reward.

As far as I can tell from looking at the rules, there is no experience awarded from Sea Encounters unless it says so on the card. In the base game, the gem that gives you experience is separate from the card you draw (you don't get XP from defeating the card, per se, but you do get to keep the gem - aka XP - in addition to whatever reward the card lists.)

The IoD rules for Sea Encounters provides an explicit list of the ways in which Sea Encounters are different from regular encounters. Earning XP in addition to listed rewards is not one of the things listed. Thus, if there is no gem on the encounter space (there isn't) then you don't get XP for it, unless the reward says so explicitly.