How you will use the boat totally drives what kind of paint you need to use. For a boat that will not be continuously in the water for more than a day or two, you are safe using a “topside” paint both inside and out. Marine enamels and one-part polyurethanes (like Interlux Brightsides) are popular but you can go more expensive with two-part polyurethanes or cheaper with household porch paints. Most (all?) varnishes are considered topside coatings. If left continuously in the water too long, these paints will soften and fail. If you store your boat on a trailer and/or pull it up on the beach at night, a topside paint will be fine
If the boat will be kept in the water for long periods of time, then you will need a good bottom paint below the water line (you can use topside paint above the waterline). If the water is saltwater, then it will also have to be anti-fouling. As the other Mark points out, if the boat is stored upright and there is a chance that water will pool in the interior for long periods of time, then topside paint may fail there also.
You will want to varnish first then paint because anywhere you have overlap, you will just see the paint. If you reverse the order, the varnish would discolor the paint at the overlap. You will save yourself a bunch or problems and work if you make sure that your paint and varnish are compatible.
On the interior floor, a textured finish is a good idea. Otherwise it will be quite slippery when it gets wet. Most companies either sell a nonskid paint or have an additive for better traction.
A year ago, I completed my Goat Island Skiff. On the interior, I used Epifanes Enamel, Epifanes Non-Skid and Epifanes Rapidclear Varnish. On the exterior, I used Epifanes Mono above the WL and an airboat paint called Wetlander below. I have used Interlux Brightsides and Interlux Schooner on previous projects so I can make some comparisons.
Both mono paints are thin and prone to run, but with a little care will result in an excellent finish if rolled/tipped. The enamel is easier to apply and gives thicker coverage but is not as hard/durable as the mono. If you have a lot of mistakes to hide, enamel may be a better choice. Overal, I prefer Epifanes because their primer is easier to use than Interlux.
Rapidclear is a semi-gloss varnish that can be recoated in after five hours without any sanding between coats. It is very thin and is easy to keep a wet edge when rolling/tipping. Without a doubt, Schooner can give a much more furniture like finish, but it is harder to apply and is a lot more work. Using Rapidclear, I was able to varnish the entire boat interior (5 coats) in two days/seven hours labor. Epifanes also sells a tinted version of the same varnish called Rapidcoat.
My choice of bottom paint was largely driven by abrasion resistance. Beach landings and rough boat ramps can really chew up normal paints. Wetlander is a very expensive two-part paint that is very easy to apply and has a reputation of being very tough. So far it has lived up to the reputation and I would definitely use it again.
