Deeply restorative and found only aboard Diamond Princess, this serene bathing sanctuary brings Japan’s centuries-old onsen culture to the open ocean. 

It’s 4pm, and the air is biting; we’ve come to Busan, South Korea, on the cusp of winter and spring. But there’s a salve for the chill. Overlooking the port city’s skyline, where mountains press up against Lego-like high rises, Izumi Japanese Bath swirls with steam, its 41°C water a welcome invitation.

The global fascination with Japanese culture has skyrocketed in recent years, and few traditions have captured travellers’ hearts like the onsen. And while the steamy facilities are found in profusion on mainland Japan, from Tokyo to the onsen capital of Oita, experiencing the ancient wellness practice while at sea is a rarity.

Enter: Izumi Japanese Bath. Measuring in at 818 square metres, the onsen facility is the largest bathhouse at sea and available exclusively onboard Diamond Princess, one of just two Japan-built ships operated by Princess Cruises. It’s both a nod to the ship’s Japanese roots and a reflection of the cruise line’s knack for delivering onboard experiences that connect guests to the destinations it sails to.

What it’s like to bathe at Izumi Japanese Bath

Izumi is one of a kind, and not just because it’s the largest bathing facility found on any cruise ship in the world. The outside pool is all drama and serenity, framed by a concentric timber pergola and spectacular views guaranteed by its location at the rear end of the ship. Unlike the indoor area, which follows traditional onsen principles including gender separation and birthday suits, wearing swimsuits is required in the outdoor tub.

As we sail off from Busan, bound for Nagasaki in Japan, the city vistas only grow more wondrous. “It’s the best part of the ship," confirms a fellow couple – the only others present this afternoon. Surprisingly, the space rarely fills up. Access comes at an extra cost, and numbers are capped at around 30, making it one of the ship’s most tranquil hideaways. I sink into the bubbling water and feel my muscles slowly relax.

The soothing effect carries indoors. Expansive windows looking out onto the ocean blur the line between inside and out, while generous use of stone and timber strengthens the onsen’s connection to nature. Leaning even further into tradition, swimwear is not permitted in this area, with the two sides of the indoor facility alternating between male and female access each day.

Watching the waves roll by, I appreciate the space’s quiet, contemplative atmosphere – a world away from the rest of the ship’s lively buzz.

Enjoy magnificent ocean views from Izumi. (Credit: Princess/Andrew Optics)

A sauna and steam room complete the experience, tempting us to linger a little longer. The heat loosens our travel-tired limbs, and I return to my room feeling detoxified from the inside out.

Guests of Diamond Princess can access the Izumi Japanese Bath for US$49 (A$70) per person for a day, or US$39 (A$56) for a two-hour session.  

More wellness onboard Diamond Princess 

Izumi is just one of many ways guests can find restoration onboard. Decompress after a day of exploring your port city and head to Lotus Spa. Here you’ll find 12 spa treatment rooms (two of which are couples’ rooms), along with a fog shower, tropical rain shower, sauna, mild steam room and aromatic steam grotto. Yes, there are bamboo massages and age-defying facials – but you can just as easily book in a keratin hair appointment or teeth-whitening treatment to feel your best.

My 75-minute aroma hot stone massage feels like floating on a cloud. It begins with dry body brushing to increase circulation, before my therapist Karen uses heated oil and stones to gently knead out muscular tension in long, sweeping strokes. 

Those craving movement should make a beeline for the ship’s fitness centre, which is fitted out with weights, treadmills and panoramic sea views. A full schedule of classes – from Pilates and cycling to barre and stretching – makes it easy to stay active at sea. And for the H2O-obsessed, the ship delivers myriad forms of waterborne relaxation with four swimming pools and seven whirlpool spas (including Izumi), several of which are adults-only.