I agree with the earlier suggestions. My idea here will not make you a better (straighter) swimmer but it may work for your situation:
Would you be able to get a set of sinking pool toys like those little rings kids dive after and place them in a line from your start and end points and use those to mark your own 'lane line'? One every 10 or 15 feet should give you plenty of feedback if you are starting to wander off. Just don't forget to pick up your 'lane' when you're done....