So, people start pretty early with rafts. A raft isn't a boat it's just a collection of stuff which floats ie is buoyant - so, with care, you can board the raft and cross a stretch of water without swimming, which is convenient. Boats incrementally improve on this by having a distinct "inside" of the boat which needn't be buoyant, separated from the outside by waterproofing. A canoe or a coracle would be examples of boats you can easily invent once you've seen rafts.

Most easy to invent types of boat are great if there are no waves. On a river there are basically never waves (yes rapids exist, no that's not common)

However at sea waves are commonplace. Situations where waves are minimal are extremely rare, usually occurring seasonally, when tides are smaller than usual and weather is calm. Sea Lion (the never attempted German invasion of mainland Britain) was predicated on absolutely calm sea because it would have used towed river barges to land troops. If there's a moderate sea but you green light the operation anyway, all your infantry drown and you've just lost the war immediately.

To be successful at sea you want even more buoyancy, to put the top of the waterproof outer parts of the boat above the waves, and you probably also want a keel, rather than having the vessel's bottom flat and sort of resting on the water which won't work well with waves. None of this is impossible, or even especially difficult with quite ancient technology, but it's not trivial, you definitely won't go from rafts to ocean-going freight transport in one attempt.