Hi Chris,
This kit was designed by the late Dave Milbourn and was his last project. Normally he would have provided very detailed instructions but I don't think he had the chance to complete the job which was finished by SLEC. I assume that there is nothing in the instructions you have concerning this?
The recommended powerplant with a brushless motor and 3S LiPo battery is pretty poweful but also lightweight. A 'traditional' brushed motor NiMH setup would be a lot heavier so it is reasonable to assume some ballast will be needed.
Perkasa is intended to be a near scale model so if you paint the waterline anywhere other than as indicated it won't look right. It also means that performance should be near scale in appearance too, this is not intended to be a model speedboat and if overpowered it may not perform properly anyway!
Did your bath test include all the internal mechanics and electrics plus all the external fittings and superstructure?
I think your best bet is to provide for removable ballast so that when the boat is complete and working you can adjust it to give the best balance between speed and appearance on the pond.
I built both of Dave's Fairey powerboat designs and reduced the battery to 2S on the Huntsman as it was simply unrealistically fast and unstable on a 3S, even with some ballast aft to improve the trim.
There are plenty of photos online of this type of craft which show what they look like at speed.
Colin