I love swimming - got back into it properly last year after watching Robson Green on the tv doing his wild swimming stuff, then I saw a programme on the British Gas open water swim series - on the strength of that I entered the Windermere mile - was going to the pool 3 / 4 times a week swimming over a mile each time, bought a wetsuit, got all excited - then the piggin swim was cancelled due to blue green algae !
Was not a happy camper.
But I have to say, I definitely noticed a carry over effect from swimming fitness to cycling fitness, my upper body strength improved, core stability improved - which made climbing much more efficient.
And my cardiovascular fitness improved a fair amount . . . I put this down to the way in which you're obliged to take deeper breaths, and hold them for longer - therefore increasing the lung volume . . . which definitely has a long term beneficial effect noticeable on the bike.
I would say to anyone who rides a bike (especially at this time of year) - get in the pool and cross train, it's not easy at first, after a few consecutive lengths you might be knackered, but keep at it, the improvements soon become apparent.
Definitely great exercise !
Hate running though so could never contemplate a triathlon.