We swam with them in November when the water was also cooler but by no means cold. There can be no guarantees as these are wild animals no matter what any tour company says. However, ask before going out when the last ones were seen. They are not nomadic animals and tend to stay in their area.

I'm not absolutely sure. I went in March but I thought we were told it was a year round activity. Other times Grey whales and Orcas?

Yes

I went on a tour March 24th and had no trouble finding a few.

They are around although they get more scarce pretty much all year except aug/sept. The peak months are as noted in other replies. Once the water warms up the are gone tho

The only time you would need to swim strongly is if the whale shark is swimming, rather than feeding. The sharks are static when feeding, so you can swim slowly around them or just float there and observe. There were young children in my group who were not strong swimmers, and they still thoroughly enjoyed the experience. I would definitely rent fins if you do not have any. That is a must. A wetsuit is honestly up to you, whether you can tolerate the water temperature. I'm used to cold water so I didn't wear one, but everyone else on my trip did wear wetsuits. I didn't get stung by any jellyfish without a wetsuit, but some people got stung a lot even with the wetsuit. The water is definitely a bit cold, but you don't quite realize it during the excitement of swimming with these amazing creatures.

Our guide said April actually due to water clarity. Just avoid the warm water summer months.