The concave shape of the interior is more difficult to smooth than the exterior. The easiest tool to use is a scraper. These scrapers are not card scrapers but more like paint scrapers. I have a selection of different shaped tools so I can work the various contours of the boat.
A sharp scraper will produce fine shavings similar to a plane, but it can also chatter and jump. If this happens it can create a serious of divots and rip chunks of wood out of your boat. I like to hold the blade in one hand while pulling with the other. The hand on the blade is there primarily to control the pressure on the wood and damp out any tendency to chatter. If the blade is sharp it should cut pretty easily without a lot of pressure. Diving grain may cause the blade to catch and jump, if so you may want to reverse directions. If you get chatter you can try reversing direction or changing the angle you hold the handle.
A mill file will do a good job of sharpening a scraper. Hold the scraper on a vice and put a fresh face on the bevel.