Repetitive multi-day dives can take its toll on your ears, with your eardrum feeling like it's marinading in sea water. While a lot of dive shops will be glad to sell you a bottle of a drying solution such as Swimmer's Ear, the discomfort of paying high prices for those little bottles can be worse than the discomfort in your ear.
Here's a quick and easy "recipe" that does the same thing with cheaper ingredients that should be accessible in whatever country you find yourself diving in. Just mix equal parts white distilled vinegar with isopropyl alcohol (preferably 100%, but use whatever you can get your hands on). The alcohol is a drying agent, sucking the moisture out of your ear. The acid in the vinegar kills bacteria to prevent infection, as well as helps with the drying.
Note: If you find that the vinegar burns, you may very well have an infected ear. Use the alcohol alone if you must, but avoid putting anything in your ear. Unfortunately this means you may need to lay off the diving.