I was thinking about the fact the character was on an island too, and figured that probably did have something to do with it.
After looking up mindflayer as a google image search, Inkay's typing makes sense for that idea as well.
If you're not familiar with "mind flayers"? They're evil creatures with psychic powers (hence the type) and are capable of mind control (the fluff says the Pokemon do this).
Yup, I'm familiar with them. Play a bit of WoW, and a few of the dungeons have mind flayers as enemies, one dungeon in specific would be Throne of Tides in Cata, in which a mind flayer creature attaches itself to an npc.
So yeah, I understand the typing and the reference. It makes sense to me now after you mention it, but I don't know if I would've picked up on it had you not said anything.
There's been quite a few Pokemon in the past that can be found both in and out of the water. The ones that immediately come to mind are Poliwag, Wooper, and Stunfisk...
As far as this goes. I guess it COULD be argued that Poliwag and Wooper both have legs to allow them to walk on land, and tails to swim in water. And Stunfisk is a fish that's also ground type, which would make sense for it to be on land as well as in water.
While Inkay is more of a squid creature, and despite it being Dark/Psychic type, I've never seen a squid (or octopus for that matter) walking around on land using it's tentacles as legs. In fact, any time I've seen pictures or video footage of squids and octopi outside of water, they just turn into a pile of skin and slime.