I guess it also depends on whether you are paddling competitions. those who do probably do regularly train.
An interesting detail may be that in September 2008 at the Dutch Freestyle Nationals the top 3 paddlers were all active/high level canoe polo players (including Casper van Kalmthout). So regular aerobic exercise does help
BTW: yesterday I received my copy of a book by Tudor Bompa, "Periodization Training for Sports". There is a lot of information in there, including training information for both sprint and marathon canoeing/kayaking.
Although I have not finished reading it (obviously) I can highly recommend reading the book and use some of it for your own training. It is out of print now but easily obtainable via Amazon in the "new and used" section (pretty cheap too: it cost me $4 to buy and $12 postage :-P ).
Cheers,
m.