As a swimmer i thought i would offer little tips for those who struggle counting 
One key thing is to do sets dont just swim up and down coz that is boring, hard to keep count and you wont work as hard. Do sets such as 16x25m, 8x50m, 4x100m, 2x200 & 1x400m.
Its much easier counting in meters then lengths. Also for 200m + swims, i break it down in my head into 4 sections, so in a 200m i'll think 100m down, work hard for the next 50, and steady for the final 50m etc, etc.
Breaking up the session and having pre-written sets are a much better way of keeping track of your meterage and keeping the concentration levels up.
I'd be happy to send people some sets if they are lacking inspiration.
Oh, and one final tip - DONT THINK! thinking or getting distracted is a sure fire way of losing count, again thats why i find shorter sets good as you dont have time to get bored and start thinking of other things.
