Smoothing the interior of a strip built boat is always a challenge. A random orbital sander usually doesn't fit the curved surface well and risks digging horseshoe shaped divots in the wood. What I find works well for me is a really sharp paint scraper shaped to match the surface, and hand sanding with very rough sandpaper.
I've used a variety of paint scraper, but I've found those with a solid steel plate blade on a rugged straight handle work the best for me. I got mine through a company that makes a "silent paint remover" but you can find good ones at Amazon. You will likely need to reshape the blade to conform to your boat. This can be done with a bench grinder or a good file. I then get the edge as sharp as I can, again with a file or bench grinder and then finishing with a sharpening stone. Grind the blade to have a slighly smaller radius than the surface you are trying to shape. I find a tear-drop shape that is rounded at both ends can be very versatile.
I originally used the paint scraper to just remove glue, but I've found that if it is good and sharp it can actuall start shaping the wood. On tight sectional curves there will be flat spots on the face of each strip. Ideally, these flat areas get shaped to a smooth continuous curve. A sharp paint scraper can take you a long way towards this goal.
Hold the handle down close to the surface so the handle is nearly parallel to the strips. With one hand lightly holding the blade, pull back sharply with the other hand. The hand on the blade helps keep the scraper from chattering. You don't need much down pressure, just enough to keep the blade in contact with the wood.
Don't try to scrub the surfac with the scraper. Pull sharping towards you, then lift the tool off the surface, reset away from you and repeat. This will keep the blade sharper.
You should be able to pull off long, wrinkled shavings that are quite thin. If you are getting dust-like bits, you likely need to sharpen the blade. If you get chatter, where the blade is hopping off the surface and then digging in a gain, change direction or try orienting the blade at a bit of a different angle so the blade no longer drops into the existing divot.
Scribbling on the surface with a pencil will give you some guidance on how you are doing. Use a dull, soft pencil so you don't create grooves. If you scrape until these lines are about gone, you should have the surface in good shape.
When you have the surface scraped down, start with sandpaper. Coarse sandpaper will do the best job. The idea of taking 40 grit paper to your boat may feel sacrilegious but it actually works well. We are talking hand sanding here. if you are able to sand all the way through your boat before you are exhausted, you are a better man than I. Going with finer sandpaper will only wear you out without achieving the desired results.
Again, a tool shaped to the surface you are working on helps a lot. I cut chunks of blue or pink foam house insulation board into a variety of handy sanding blocks that can be wrapped with sandpaper. If you are having trouble finding coarse enough paper, look for belt sander belts. They last a long time and come in agressive grits.
Some more scribbling on the interior with a pencil will again provide feedback on how you are doing. You want to eliminate any overhanging seams that will not fill with epoxy when fiberglassing. Feel the surface with your hand as you proceed.
Sand parallel to the strips, working systematically from sheerline to the centerling and from one end to the other. It is helpful to vacuum out sanding dust as you proceed. This will keep the dust from interfereing with your work and reduce the amount of dust flying through the air.
I was applying carbon/Kevlar cloth to the interiour of the boat in this video so I did not need to create a scratch-free finish. I stepped up to 60 grit sandpaper and then stopped.
You may be able to use a random orbital sander with a countour pad after you have leveled/contoured the surface with the coarse sandpaper, but this will depend your tools.