Having been actively involved with diets, ingredients and their problems, I can assure you there is a very good possibility that your dogs ear infections are a product of an allergic reaction to either something in the environment or his diet. I would be the diet, having had several dogs with this problem myself which is what got my interest in learning about ingredients and how they can affect a dogs body.
A dogs immune system can be lowered by eating a food that is disagreeing with him and thus leave him open to other problems such as itchy skin, low thyroid, ear infections, staph infections, as well as many other conditions affecting our dogs. When I find a dog that is have a recurring problem that isn't getting completely eliminated by medications, then it's time to look for another reason and the food the dog is eating is a good place to start.
Get your lab off all corn, wheat, soy, by-products and chicken and see if that helps. This includes treats as many of the treats out there have way too many grains in them and that is what you really have to stay away from.
Lots of dogs do a great deal of swimming without ending up with problem ears, so that can't be the whole answer. Something else has to be invovled and I'd start by getting your dog onto a good quality kibble.
As to administering ear drops, put the dog in a sitting position, place your legs on either side of his body so he can't move, grab an ear and have a filled dropper ready to use. Quickly squirt the drops into the ear and message the bottom of the ear to make the drops go deeper. All done.
Sometimes it takes up to 8 weeks for a changed diet to make a difference, but if the ear problem is coming back, the next step is a thyroid panel, checking both T3 and T4 along with the circulating thyroid to make sure the dog is not Hypothyroid which would cause a lowered immune system.
Good luck, find the cause and you won't have to worry about the symptoms, but I don't blame the swimming completely.