It is probably hotter than Odessa given that it is located further south but it is not unbearable. There are a lot of beach resorts in the vicinity that are not far from Varna. I would recommend staying there. As far as the hotels - there are literary hundreds of hotels in Varna and the vicinity. Also Varna is quite popular destination. Check out Albena, Golden Sands, and Saints Constantine and Helena. There is hotel for everyone. Also the beaches are really nice and the water gets warm. Most popular times to visit - July and August.