Swimming in lanes is not difficult
You go up one side, across the end, down the other, across the end and repeat. If someone is going slower you wait for a convenient moment and overtake. If someone is going faster you help create that convenient overtaking moment. When everyone knows what they are doing and acts with a bit of courtesy it all goes swimmingly (ha ha) even with quite a range of speeds.

Unfortunately, every pool has its twats. Normally these are old, fat or both and they are almost invariably slow. They will. not. be. overtaken. under. any. circumstances. and they will go to considerable lengths (ha ha, again) to obstruct anyone faster than them.

Yes, fat man with the muffin top and the blue earplugs who wobbles his way back and forth along the medium lane at Dalry Pool in Edinburgh at 7.30 every morning, this means you. The two old dears with the yellow and white swim caps aren't much better.
(ubergeekian, Mon 16 Apr 2012, 10:27, 3 replies)

and then you get the bigger twat
who wants to swim widths
( sandettie light vessel automatic New Twitter - @bollocksreally, Mon 16 Apr 2012, 10:36, closed)

Way back when...
The women of a certain age [and senility] who'd get their hair done, and the proceed to lope up and down the pool at a pace which would make snails complain about speeding, and shoot venomous looks at anyone who nary disturbed the surface tension of the water lest it mess their new 'do up.
(caescarna Hell's Roomba, Mon 16 Apr 2012, 14:04, closed)

There are a couple of them at Dalry too
who swim very slowly up and down the open water bit of the pool, side by side, chatting. They get a bit cross when you swim between them. Repeatedly.
(ubergeekian, Mon 16 Apr 2012, 23:17, closed)