Stills aren't hard to make but I'm trying to picture a huge pot that will fit in your yak. None of my ocean boats have bulkhead big enough to hold a 2-3 gallon pot and thats what you'll need to make it worth while. You'll also need a chiller coil of copper tubing, the longer the better, and a hood over your pot to connect the coil. You'll get way more than "cups per day" but it's work. Coil stores in your pot when traveling.
You might check out reverse osmosis filters. Expensive but smaller.