It looks like a Slime with a Shellmet permanently stuck on its head, and is often encountered when sailing. Though not as common as their land-lubbing cousins, the Seaslimes are infamous for boasting high defence and elemental resistances, as well as turning tail when battles look bleak, not unlike the Metal slime family.
Seaslime appears as a C-rank monster of the Slime family as part of the limited Dragon Quest III event. It can be recruited from Chapter 2, Episode 1: A Ship Voyage and Chapter 3, Episode 1: Another Ship Voyage of the event quests at any difficulty, as well as Chapter 16, Episode 6: Sightseeing at Port Town of the main storyline. The seaslime can participate in the Armful and King kelp's Battle Roads as a party member.