I'd say it depends on how busy you enjoy being. You would have plenty to do (and some time to relax.....one of the benefits of a day at DC in the midst of themepark-centric vacation) with just the dolphin swim, BUT.....if you're a go-go-go type of person, then you could absolutely do an additional activity in addition to your time with the dolphins.
On a first visit, if I were you, I would take the additional $$$ you would spend on the 2nd activity and devote those funds to purchasing your dolphin swim photos......and leave yourself extra time to enjoy snorkeling in the Grand Reef, floating around the Wind Away River, feeding the birds in the Aviary (which I assume will be reopened by the time you visit) and soaking in the lounge chairs in the Freshwater Oasis with an adult beverage and a warm chocolate chip cookie (or two, or three.....). It's a luscious day when you can just kick back, soak up the sunshine, and savor the serenity.
That said, we LOVED both the Flamingo Mingle (which we did in September) and Seaventure (done earlier in 2021). Both were fabulous experiences and well worth the cost. I will note, though, that we did those add on experiences on visits when we didn't choose to do the dolphin swim. We don't tend to "overbook" our DC days so that we are 'forced' to dial down and enjoy the chance to mellow out for a day, especially when our other vacation days tend to be at a much busier pace in the theme parks.