This isn't the first case of an aquatic creature-based Pokemon not being a water type. First that should be noted is the Psychic/Flying Lugia, which lives in the sea but isn't based on any real life creature. Stunfisk and the Tynamo Line from Gen 5 Dragalge from Gen 6, in addition to Crabrawler as you mentioned, all break the convention of Pokemon based on real life ocean creature being water types. As to why this is, we can't know for sure until Gamefreak states a reason, but we can speculate. It may be because the Grapploct line spends some time on land, as noted by its Shield dex entry:
"Searching for an opponent to test its skills against, it emerges onto land. Once the battle is over, it returns to the sea. "
I think the closest parallel to Grapploct is Dragalge, the only difference being Dragalge evolves from a water type (Skrelp), whereas Grapploct does not, but they are both Pokemon that live in the ocean but aren't water types. Another Pokemon worth mentioning is Dhelmise, which is ghost/grass despite being found in the ocean. It should also be noted that while Grapploct isn't a water type, it has multiple water-related qualities. It can learn many water type moves via TM and TR and is in the Water 1 egg group. So despite lacking a water typing, it still has many aspects relating to water.
td:dr an ocean dwelling Pokemon not being water type isn't unprecedented and Grapploct still has water-related qualities