Welcome aboard, Melissa! I'm so glad that your whole family is looking forward to swimming with the dolphins while you're in port at Costa Maya! My family will be doing our first swim with dolphins when we cruise with Disney later this year too!
Normally, yes, I'd suggest doing the
Dolphin Swim because guests get more interaction with the dolphins, but since that starts at age eight in Costa Maya, your five year old cannot join you for that. While he can absolutely participate in the
Dolphin Encounter, this Port Adventure does require that children ages three to 12 be accompanied in the water by a paying adult booked on the same experience. Sometimes the Swim and Encounter adventures will be at different times or different areas, so you could not send him to one while you're on the other. Perhaps either you or your husband could do the Encounter with him while the other takes your 17-year old to Swim. Either way, you're sure to have an amazing experience interacting with those beautiful dolphins!
Please let me know if you have further questions as you continue to plan, Melissa! I'm happy to help!