Do a fair amount of boating/sailing and have used dry bags for decades. They are a pain to pack/unpack and the item you're looking for on the road is always at the bottom. I use them all the time and do like them being essentially waterproof. The downside is moisture... inside the bag. They are waterproof, so if you don't go for a really expensive breathable material like GoreTex or something like it, your stuff can get pretty musty. Throwing in some desiccant packs helps a lot (get them on Amazon).

I like the Wolfman dry bags (have them in black and yellow) but they are BIG and will scratch up the paint if your not careful. They also interfere with the saddle bags. This is a 35L $60 bag I got recently on Amazon. Decent quality for the money and it fits pretty well on the bike. You can get two if your two up and stack them. No arguments on who has more space. The bag from my prior post will be the go to bag as it is much easier to use on the road. Comes with it's own rain cover. Anything in the bag that has to stay dry I put in its own plastic ziplock anyway. I'm a real fan of ziplock freezer bags. They are a cheap waterproof compression sack. Put stuff in, roll it to get the air out and click/zip. Then use a Sharpie pen to write on the bag what's inside. Good for a trip then throw away (call it carbon sequestration for the global warming crowd).