Great idea, go for it!
I asked my wife 22 years ago in just that manner.
We were diving in St. Croix, in about 30’ of water.
While on the bottom (sandy area) I motioned for her to stay still while I wrote the proposal on my slate.
I asked her to marry me, and gave her 2 options: yes with a huge box to check off, and no with a microscopic box to check off.
When she saw the message she was grinning so much she almost dropped the regulator out of her mouth. Needless to say she said yes.
Upon surfacing I took a picture of the slate. It has since been framed and is hanging in our house.
Best wishes!