Good to hear from a fellow Sun Sporter. The brand seems to get a bad rep on here but their identical to the Honwave and made in the same factory.
Anyway, just to confirm what Peter said. The Sun Sport manual states 0.25 for the tubes and 0.8 for the floor. Anything less in the floor and it compromises the ride and effects speed and handling. And also Landlocked's advice is worthy of note.
I've had my 3.2 for a couple of years now and it's been great fun however it's crucial that the floor is 0.8 when it's in the water. By that I mean if you pump up the floor to 0.8 and put it in the water, the cold water will of course lower the temperature of the air in the floor and so will lower the pressure. So a nice rigid floor on land can turn into a floppy thing in the water. My routine is pump it up to 0.8, put it in the water for a few minutes until the air in the floor stabilises i.e falls to the temperature of the water and then pump it up to 0.8 on the water.
This is of course a lot easier with an electrip pump and a small battery as the foot or hand pump is a bugger of a job on the water. A bravo pump and 12v battery is an essential investment if you're inflating and deflating on a regular basis.
I get 17 mph wot with a merc 6hp so I would imagine yours will fly with the 9.8.
Hopefully I've attached the relevant manual page.