The Spa at Grand Prince Hotel Osaka Bay
Hyatt Regency Osaka's spa is mentioned by expert writers. Out of 2 publications that we monitored, 2 acknowledged the spa, including Lonely Planet and Fodor's.
Grand Prince Hotel Osaka Bay 
Lonely Planet
Fodor's
Lonely Planet "Walk into the spacious, elegant lobby here and you'll immediately feel relaxed"
Fodor's "Modern comforts abound: the guest rooms are spacious, especially deluxe doubles and junior suites, which are larger than the typical Japanese apartment"
