Lerasp - given your wishes to dive and to eat out at restaurants outside the hotel, I suggest that Amanusa might suit you better although I don't like the Nusa Dua area as it's an enclave of hotels with little Balinese culture. You would however be closer to Uluwatu and Jimbaran and would be able to dive by taking a boat to Nusa Lembongan from Benoa Harbour.
Amankila is in a more remote area so you would be more or less confined to the hotel in the evening. You really should stick with your plan to stay in Ubud.