Good, expensive dry bags are indeed as dry as the name suggests. I once was on a raft and kayak tour in scandinavia. We had everthing in those bags (food, foto equipment, clothes, etc.) and there wasn't a single drop of moisture inside, though there was a lot of water and one actually fell into the river. The nice thing about those was, that there was still enough air inside to keep them afloat, even with heavy stuff inside. With such expensive equipment, I'd put some silica inside, just to be safe and bind any moisture from the air.
Pelikan cases are good too (some of those are designed to withstand explosives and tsunamis!), but I think, the bags are more practical and probably a lot cheaper.
...but I still think, it's crazy to carry LF gear on such a tour. I'd choose something lighter and faster. At least carry one 35mm or small medium format camera, just in case you want to take photos from the boat.