Normal deep one is pretty underwhelming. You get 1 stack of smelly I believe (makes you easier to detect). You have blue eyes. I think the chances of your offspring change more towards fishfolk/deep ones.
If you complete a certain task and awaken the Sea Witch, while you are a deep one, she gifts you an ability that lets you transform into what I can only describe as almost like a Zora from the Zelda series. This gives you a new set of pretty powerful abilities: magic water ranged attack, a mist that pretty much turns you invisible, and the ability to summon two female deep ones that help you kill everyone in the area (their power scales with WIS and they can be very OP).
There are also downsides to transforming into a Sea Witch/True Deep One. You get a lot of debuffs if you transform, you can't wear most clothing or use weapons while playing as a True Deep One. All species will try to rape you on sight as well while you are in that state, due to pheromones.
Sorry if I'm inconsistent with the naming here, the transformation is sometimes referred to as both True Deep One and Sea Witch.
If you want to unlock it, rebirth as a deep one (you'll know you're a deep one if you have blue eyes instead of the usual greenish color). Go to Fishtopia island. Go to the old temple to the east of that island. You will find a puzzle that grants entrance to the temple. If you go North from the ladder after entering, you will find the sleeping Sea Witch. Either give her a bunch of human semen or go to the bar off to the west of the island in one of the towns and you can recruit a human somewhere in there that wants you to bring him to the Sea Witch. After she is satiated, you can speak to her about various things and unlock the Sea Witch transformation.