From the OP's description, a good quality dry bag to keep out rain and water spray when under motor power seems like what he is asking about...I could be wrong.
That said, dry bags and submersible dry bags are two different categories for me....
When talking dry bags, I've relied on several varieties/models of Seal Line for many years...no complaints whatsoever...excellent bags.
However, over the years when it comes to submersible dry bags, it's Watershed all the way for me. I briefly flipped my canoe one year, and the dry bags I used took water...my fault for flipping the canoe, not complaining about the dry bags that were well sealed, and fortunately I had some of my stuff double-bagged (a dry bag inside of a dry bag). However, I am absolutely confident that a Watershed bag would have kept every thing dry inside all by itself.
On week-long remote and long-distance packraft floats, all of my absolute necessities are in Watershed bag(s)...and then other items are in Seal Line dry bag(s).
Edit: This summer I'll be trying out a couple of zip Seal Line dry bags that they claim are submersible to some degree...but I'll still be bringing my Watershed bags.