I was in La Paz and La Ventana in late April thru May 10 this year snorkeling and freediving. Worth noting:
1. Water temperature at the surface was about 24-25C (75-77F) - not 21C. Water viz was about 10 meters at worst and maybe 15 meters at best.
2. I agree that April is not the best time to dive in Baja. Water is cold and viz is poor. I would not bother with Cabo San Lucas unless you want to party and get drunk. Cabo Pulmo has good diving but I would not dive there in April.
3. April/May is not whaleshark season, and even if it were, the water inside La Paz Bay, where they usually hang, around is green and bleah. A better place to do whalesharks is Isla Mujeres, where the water is blue, but the whaleshark season there is during the Summer months.
3. There are sea lions but in April, they are grown up and less fun. It is the juveniles that are most playful and interactive, and their season tends to be towards October/November. Other than this, I don't particularly care for the diving in the La Paz area, altho it can be really fishy.
Having laid out above all the negatives of diving in Baja in April, I now add that:
4. This year, there were mobulas (not mantas) - pygmy mobulas (Mobuka munkiana) to be specific - in the hundreds and sometimes thousands in the channel between Espiritu Santo Island and the La Paz, and between Cerralvo Island and La Paz. I can't vouch for next year, but I plan to be there again in mid-April 2023.
5. During this time of the year, and with a lot of luck, encounters with whales of various kinds - Blue, Humpback, Fin, Sperm, Bryde's and Killer - is possible in these waters. However, Park regulations prohibit swimming with Baleen whales - that would be all of the above except Sperm and Killer Whales, which are toothed-whales.
6. There are also encounters with dolphins, sometimes in the hundreds, but getting in the water with them is doubtful at best.
7. The difficulty that you, as a lone individual, will encounter in trying to go out to see mobulas or any of the whales is that for most operators, the mobula season starts on May 1st. Some might be persuaded to go out before that, but not without a minimum of 4.
Lastly, from my perspective and biased opinion, just the experience of encountering any of #4 or #5 above blows away anything that you can see while diving in the Yucatan. Diving cenotes does nothing for me because it is marine life that interests me. So it all depends on what you wish to see.