This hotel with complimentary shuttle bus service to and from Atami station and a tower building offering all rooms panoramic ocean view is a good place to stay to watch the fireworks. The buffet dinner is quite good too, with unlimited crab legs and tempura (including zuwai crab legs).
Rooms in Misaki building are much cheaper but they do not have ocean or fireworks view but you may squeeze with everyone else at the lobby on Level 2 to view the fireworks hahaha
Book at least 2-3 months in advance if you want it for a fireworks night.
Rate: JPY50,000+ (SGD$800) per room per night (include buffet dinner and breakfast) for tower building. On non-fireworks nights, you can book room only or B&B.
Access: Take the train to Atami station and catch the complimentary shuttle service (half hourly but do check the timing with the hotel)