Yep, phone the airline as I have found it varied depending upon who you fly with. Some will allow you class Scuba gear as sports luggage in much the same way as they allow you take your golf clubs on holiday and wont surcharge you - others wont and require a supplement which can work out quite pricey!!
Two that spring to mind that I have used and havent charged are BA and Air France, although BA did manage to loose my gear BOTH directions so it got couried both ways. Got better treatment than I did, wonder how much that servie would have cost if I had asked for it!!!
