Help on replacement mattress for boat

Hi, really impressed with your website. Some advice please. We have had Vi Spring mattresses at home for many years - specifically a Regal Supreme zip-link super king for the last 5 years - and are very happy with them. Have recently bought a boat (inland wide beam narrow-boat) that came with a standard king size (5') bed and 'quality' mattress. But it doesn't compare to what we are used to so we intend to replace it. Obvious choice was another Vi Spring but your products caught my eye as an alternative. We are both 5'10 and about 70-80 kgs; medium springing with Vi Springs has worked well for us. The mattress will sit on a well-ventilated slatted base (pretty solid slats), built in to the boat. Getting it in will be tight - I have measured the diagonal across the main door as 5' so it will be a squeeze!

Any advice on a suitable mattress for us much appreciated. Many thanks. >