Can I sue for an injury I got playing a recreational sport in Ontario?

It depends heavily on what actually happened. Participants in a recreational sport are generally taken to accept the ordinary risks that are simply part of playing that sport - being tackled in football, hit by a puck in hockey, or falling while skiing - so a claim over an injury from that kind of normal, expected contact typically does not succeed, since it is not a breach of any duty for another participant to play the sport the way it is ordinarily played.

That said, a claim can still succeed where the conduct went beyond what is genuinely inherent to the sport - reckless or deliberate conduct clearly outside the rules, for example - or where an organizer or facility failed to provide reasonably safe equipment, supervision, or playing conditions, which is a separate question from another participant's conduct during play. Signed waivers are also common for organized recreational activities and can further limit claims, but like other waivers, their enforceability depends on how clearly and specifically they were drafted and presented. What counts as an inherent risk depends on the specific rules and customs of the sport involved, so the details of exactly how the injury happened matter a great deal.