Only you can answer this question. All I can do is give you information upon which to base your decision.
Swimming with whale sharks in the Bay of La Paz is highly regulated. The giant bay is divided into three zones – a recreation zone, a marine traffic zone, and a 7000-hectare reserve zone for the whale sharks. The animals are not restricted in their movements in any way – it is open ocean. Here are the rules that Oliver mentioned to us: all boat captains and guides must be certified via a training course. Boats cannot exceed 40 feet in length, cannot exceed the speed limit, and can only operate between 9am to 5pm. One boat per whale shark (there are normally 10+ whale sharks in the allowed area), with a maximum of 14 boats at a time. Oliver told us they once had to wait 4 hours for their turn. No more than 6 people in the water at a time (as mentioned, we did three guests plus our guide and alternated). Personal rules include no touching the whale sharks, no flash photography, no harassment, two arms distance minimum, mandatory buoyancy device, no free diving (surface only), biodegradable sunblock only (provided), and no big splashes – the more you blend in, the better.
The whale sharks are free to dive or leave the area at any time, and they are more than capable of out-swimming even the fastest swimmer, if they so desire. I will add this; our guide Oliver was not only knowledgeable, but his genuine respect and concern for the whale sharks was evident in everything he did from the way he talked about them, to the way he enforced the regulations. It was clear that if I got out of line, I would have to deal with him.