Minecraft is all about exploration. You can explore it by land and sea.
Horse is useful when you exploring by land.
But sometimes (very often) you need to go through ocean.
And that's where pain starts... You can not put horse in the boat. You use Lead.
But you shall not go fast by boat when you hold Lead, because Lead can break and you lose your Horse.
You need look back to see: is your horse still on the Lead?
I suggest to add new, larger boat (2x3 size). You can put your horse in, you can place chest to keep your pressures loot.
Kind a floating base.
P.S. Maybe you can face pirate boats loaded with hostile mobs?
I suggested longboats and sailboats in my Ocean Additions suggestion. I think larger boats should look also be able to hold chests. I don't think they need to be any wider than the existing boats, though; in my suggestion they are just longer.