I like the idea of this. However, I feel should be a full boss. Don't Elder Guardians have 80.. err nvm, you said hearts, so that's 160 health. I was about to say. I'd say just give it more health and make it a full on boss to the Underwater Monument's Elder Guardian mini-bosses. Support.
Agreed( with support!).
EDIT: I think I broke my signature
While I fully understand you mean Prismarine Horn, as in musical instrument, and Hydra horn, as in a hard growth atop the head, it seems a bit redundant don't you think? I'd recommend changing it to Hydra Heart and make the heart it grants permanent for as long as you possess it within your inventory.
Restrict it so that you can only have one in your inventory at a time and it just becomes a nice little boost rather than something you can potentially farm to acquire an ungodly amount of HP. Seeing as how it's a boss, it needs to grant something fairly good...otherwise why bother? A temporary boost, in my eyes, just isn't worth the fight...but a single heart permanent boost would be a nice trinket.
Either way, support. We need an Overworld boss.
Make it so you have to summon it in a swamp biome...that makes sense, no?