Unsurprisingly, most people come to Miami in the summer. But, as a Miami native, I would advise against this.
First of all, it’s peak tourist season, so everything will be crowded and overpriced. Plus, the heat this time of year is almost unbearable, especially for people who aren’t used to the tropical climate. And, most importantly, summer is hurricane season, so you always run the risk of having your plans canceled by rain, storms or, yes, hurricanes.
Spring and fall are by far the best times to visit the city. The weather is warm enough that you’ll enjoy getting in the water, but there’s a smaller chance of tropical storms. You’ll also get off-season prices.
As for “winter,” it really depends on your climate preference. Though it never gets actually cold in Miami, the temperature can drop to the 50s if there are snow storms in the north. If you’re comfortable getting in the water in that weather, you’re very much welcome to do so. Just know that there won’t be many other people as brave as you.