Although the Seven are considered the greatest superheroes of the world, some are thought to be greater than others, and thus there is a dividing line between the Big Three, and the lesser members of the group such as the Deep. Neither as lucrative, nor as powerful as Black Noir, Maeve or the Homelander, the Deep and the other members of the seven are less of a concern to Vought American or the Boys. Furthermore the Deep has no particular connection to any of the series protagonists, so he hasn't had as much focus as the Seven's other lesser members.
The Deep's backstory and personality haven't been expanded on to any great extent, though it has been established that he was created in a similar manner to the Homelander, and he comes across as quite serious-minded and reserved. His speech is formal, and sometimes given to grandiose flourishes. He has stated that he can't remove his helmet due to an Atlantean curse, so he may be delusional.
The Deep hasn't displayed as many of the decadent or murderous tendencies of his colleagues, though this doesn't mean he's particular noble. He has shown no desire to help others, and quarrels about how much money he makes.
The Deep can fly and has demonstrated superhuman strength and durability. While he has been seen underwater, and bears an aquatic theme it hasn't been established if he has any nautical-based powers.