Dragon Tears, says it gets 10 feet tall and 8 ft wide.
My hemlock is enormous, and has large open area under neath, one side broke out in a bad ice storm years ago so it's kinda lopsided. I wondered if I could plant this small tree under it, would it do okay? It is close to a water spout, as I know it will need supplement water probably being under the other tree.

If you manage to dig a hole - Hemlock roots are shallow and matted - the Japanese Maple will struggle for nutrients and water. Hemlock trees have very aggressive root systems so any extra care would be grabbed by the greedy Hemlock. I doubt the Japanese Maple could complete.

perhaps I'll try digging the hole out before I buy the tree, that might be a good idea. I do have a hydrangea and some boxwoods there that seem to do okay. They don't grow alot but they look nice. I do water them extra, I have never fertilized them though. I won't mind if it doesn't grow alot, as long as it looks healthy. I was thinking the root competition may keep it from growing too large for that spot. Although the space is pretty good sized.

If I was attempting this... I would expand that bed...

Attempting to place tree as close as you've drawn... would seem doomed to failure as lucy68 pointed out...

But if you got rid of some more turf... Or better yet, expanded all 3 beds so that that patch of turf in the middle was gone... I think you would have a happier island, whether you planted the maple or not... you would certainly have room for the new tree...

Hmm... Seems like you have a different side in mind... Still, the same advice... expand the bed... all the way around... I think you'll be happier, and the maple would have a better chance out away from that huge hemlock...

What kind of watering setup do you have there?
I'm kind of wondering whether it will even be possible to keep the new plant watered enough... Those island beds where everybody wants to plant... tend to be dry...

unfortunately the water system is me

Yes the soil here stays pretty dry if I don't water it. I was just thinking maybe a small maple would do okay since they do have vigorous root systems. I don't think I could link the two beds as that is the only area to fit the mower in to mow the back half of the yard that we just have trees/grass. The garden area in the back is large, but we do still have some that we mow.