I carry more than one bag with me or close to me on most trips. I too have a Salamander retriever bag, and I love it. I always boat with. I never really intend to use it as a boat recovery device, but it would work for kayaks and some raft issues. I carry another Whitewater designs big bag with 3/8" line and about 65 ft of rope in that. I really like the bag mouth size, it is easy to repack. I don't re stuff the bag until it is not needed, I've learned to throw coils. This one goes in the raft or IK also. I'd use it to unpin a raft if I couldn't get to my 200 feet of waterline. That rope I take only in my raft on most of my trips. So in my kayak I carry a total of 130' of rope and at minimum 330' in my raft, and I typically have backup bags in the raft as well. The newer retriever bags have a panic ball on the buckle, a good thing if you use it to tow with. I throw my big bag really well, I should probably toss my retriever more in practice.
I don't think 75' of 3/8's around your waist is a realistic/ practical goal. If you need that much rope, keep it in your boat. I love waist bags, and I don't boat without mine, but I recognize it's limitations.
Wishing I was on the river instead of surfing the web...