I swim at Tenacatita at least once a month, and sometimes every Saturday or Sunday. I was there yesterday.
It's always a very good walking beach, and almost always a great swimming beach. However, I've gone through six foot surf there (very rarely and only in the summer), and I've seen giant manta rays -- not dangerous if you don't surprise them. I've actually been asked to swim closer in, by one of the weekend life guards when he saw mantas playing near where I was swimming, about 100 yards off shore.
There are generally three guards on duty -- two under a sun cover, a little south of the entrance, and one watching the house in the northeast corner of the bay. They are nice, friendly guys. Ask for their permission (to be polite), if you intend to drive to the south east corner of the bay. They will tell you to stay out of the ruins of the houses. This is for your safety, and they strictly enforce this rule.
There are a few tables for rent (toward the north end of the beach) with umbrellas for sun cover, but on busy weekends, they go quickly -- so bring an umbrella.
The best swimming area is usually toward the northern corner, because there is protection from the prevailing northwesterly winds. In the summer you can get big rollers in there when there are tropical storms passing from the south.
If you are not an experienced ocean swimmer, swim there in the winter in the morning or early afternoon (before the wind comes up) within a couple of hours of the low tide.
When the jelly fish are there, you will know it by a quick walk of the beach, If you see some drying out on the sand, they're there. They're more common in the middle part of the bay, not in the better swimming area to the north. Yesterday, there were none.