A few experiences/airlines from recent trips...
Turkish Airlines: London - Erzurum, not too bad. Happy to accommodate boats for a small fee (£15)
Qatar/Gulf Air: London - Kathmandu, very good. Both happy to take boats, even when we a groups have pitched up with 12 boats (and one mountain bike). Having said that I did pre warn on a couple of trips and they said it would be ok - but they wouldn't give any written confirmation to that effect. But as it turned out - all was well.
Iberia: Munich - Ecuador. Not too good. Despite no other gear except a boat they were having non of it. $50 supplement for a 'surf board'. Hey, ignorance can work in your favour sometimes, as I guess a kayak would have been more. BA from London - Ecuador was no trouble.
Easyjet: Zurich - Leeds, Very bad. But we did have 140kgs of luggage between 3 people (mostly snowboarding gear). Big ecxess charges (£250+) but they got so confused with us changing what bags were going where (it wasn't a direct flight) they forgot to charge us!, And we didn't bother reminding them.
Easyjet: Ireland - Alps (from another post). Very bad. Difficult to swing it with the boats by all accounts and an extra £43 each.
Royal Air Moroc: Marrackech - London, ok. Just. They do impose size limits on luggage and a boat exceeds that. But a friend recently blagged it and no problems. Though they could have easily told him 'no' or made him pay.
BA: London - Anywhere, The Best!! Had no problems with BA at all (so far!). Always just pictehd up and checked in, pointed at the boats and told them roughly how much they weighed and that is generally that. Even been sent to the first class check in a few times (but sadly no first class seat).
Hope this helps,
Ben