Unless you had a bit of shade cast over your pond where your floating islands are anchored you will want to pick a vegetable that is sun happy. There often isn't too much shade in the middle of a pond.

Too much water is a big killer of house plants and gardens. I use only above ground smart pots for 90% of my crops because I can almost guarantee I will never drown anything when using these pots. You could integrate these into a floating island, compensating for the added weight of the 12" of dirt in the smart pot(s).

The biggest PITA for me would be the almost daily interaction required. Every day we weed and remove insects and review the condition of the crops. So that could be one reason why growing crops on floating islands hasn't taken off - having to get to that island on a daily basis to do the required work. Unless of course you just put them out there and hope for the best.

BTW a potential huge benefit to growing on such an island would be the possibility of having very little issues with insects or rodents.