I have been Stand Up paddling for a while and have a few solutions out.
I use a standard leash that velcros to my ankle but I take the velcro and fold it back on itself. By doing this I only use an 3/4 of inch of the velcro. This keeps the board with me but it not enough velcro that it would hold me if it got tangled on something.
The thing about the leash is that I would only recommend it for class 4-5. I have found that is sometimes gets in the way. Secoudly it makes all the differance to just hold on to your gear when you swim and the best part is that you can just climb back on.
Becareful with the T grip...I broke three teeth surfing in the ocean in South Carolina a month ago becauce I hit myself in the teeth with the t grip because I took my hand off in a rought situation that just got worse when salt water hit the open nerves.
This is a really fun sport and if you kayak it makes it that much better. I have two stand up paddleboards in my quiver of kayaks, surfboards, and kiteboards.