For a boat
your size, I'd say 5/16" HT. Preferably half a barrel or so, AKA 275', backed up with at least 100' of your favorite type of rope rode
. Minimum strength around 15k (wet that is).
If you hunt, you can find the chain for just a bit over $2/ft. And at that size, you're looking at about 1.1lbs/ft, so #300, plus the rope rode