I second Mark’s opinion. It’s safe to open the top cover of the Boston valve on the water because of the one-way valve inside, and you can top up through that. Bruce from Fairbanks carries a ~1 meter length of 7/8″ OD silicone tubing to plug into the Boston valve so he doesn’t have to twist around to get his mouth on it, which is a good idea. I can do it without the tube, but it takes a fair amount of flexibility. If you’re paddling with a partner, it’s a bit easier if you raft up and top up each other’s boats, and in many cases it’s just as quick to go ashore for a minute and top up there.