Because of some problems we've encountered in our travels, I've been trying to come up with a simple and inexpensive way to insure my spinning rods wouldn't get lost if our kayaks should tip over, if they got caught on a tree, or if one got snagged by a cast. All these incidents have happened and I lost a rod/reel (well over $100) in Briery Lake in Virginia and almost lost a brand new spinning rod in the Au Sable River in Michigan. I can't remember how many times I've flipped a kayak getting in or out, but that's usually in shallow water. The point being that kayakers can be prone to loosing rods occasionally.
I considered a number of ideas that were too complicated to be used every time, and I think simplicity is essential. My idea is to take a piece of foam “Pool Noodle” about 6-8 inches long and split it half way through so that the rod can slide through the slit and into the hole. Put it between the reel and the first line guide. the natural springiness keeps it on the rod.* I've tried these with my salt water rods and they float with the reel down (see photo) and the rod sticking up above the water. I showed it to some other kayakers by throwing the rods into the motel pool and they thought it was a great idea and got some pool noodles that evening. I tried it on some of my smallmouth rods (light and ultralight) and it worked, but the reels (1000 weight) didn't pull the rod upright but they floated flat on the water.
When fishing, we just take the foam off and lay it close by, then put it back on before putting the rod back in the rod holder. Most of our rod holders are flush mount type and positioned behind our left sides.
Bait casting rods may or may not work because of the size of the line guides. I'm sure I can modify the “Pool Noodle” float to work, but BassPro sells some rod floats with velcro straps...so that would work for those. I may also add a carabiner to a noodle float and clip it to the first line guide. Or, I can put the foam on the rod handle, close to the reel but leaving enough handle to go into the rod holder. Again, simple and quick.
I'm always interested in serious ways other people have solved this problem.