As far as I know it is not illegal to own a speargun in Ontario nor it requires any registration. Gun control regulation seems to be under federal authority and I suspect is the same in all provinces. I bought spearguns overseas and have them delivered in Canada (Quebec) without any trouble. I had to pay customs duties equivalent to the sales tax here (15%).
If still worried have a look at the Canada Border Services Agency site and on particular at documents concerning importing non-commercial goods (see for example:
http://www.cbsa-asfc.gc.ca/E/pub/cp/...rc4051-e.html). I doubt they have anything specifically allowing spearguns to enter the country (and yes, this doesn't make it illegal as Strangelove mentioned). You may also contact them by phone and ask for clarifications.
Good luck.
Virgil
