Don't think you have much experience with sea creatures?
What do I do with my grandson? He's extremely jealous of other Mortys that get to adventure no matter how many times I tell him that he'd dry up and die on dry land.
I'm already working on something to fix that, but he's impatient and getting on my nerves.
First I want to say thank you for reaching out despite worries that I may not understand. Deciding you need help is an important step, and can be an extra tough one when you have differences in noteworthy areas. While I may not be familiar, I can try my best to empathize and give advice from a professional standpoint.
As for your grandson, I can see how he may feel jealousy when compared to others who I'm told have many similarities to him. Kids are very social creatures, they work by mirroring everything around them, so diversity is an important aspect of this.
I suggest taking your grandson on your own adventures, focus on what he can do rather than what he can't. You need to show him that there are perks to his life as much as there is anybody else, and it should be because you care.