The Americans actually did, from my recollection of production figures and manpower, actually have quite an impressive shipbuilding industry and sailor base. But it clearly was difficult to actually put this into effect of building up a regular fleet, based on the war happening in the US, the overwhelming superiority of the British navy which meant that starting a build up was difficult, that the US was just being formed and it had to work out the governmental institutions and organizational structure of its navy, and above all else financing it. And most importantly, there was simply not time: it takes years to build a real battle fleet, and so even in perfect conditions, it would be years before the US would be able to form a fleet capable of a stand-up fight with the Royal Navy (and even then, there is no guarantee, given that the Royal Navy would be significantly larger, more powerful, and experienced). If you have the US win the war on land early then peace will come and there will be no need for a fleet, and the organizational and infrastructure developments for it will never take place, while if the US is bogged down and forced to develop a more powerful government and has a real need for a fleet, the more pressing needs of the land war will prevent it from being formed. Either way, a real fleet is low on the priority list for the US during the war.
A real US battle fleet beyond a few ships of the line (which the US did build a few of in the 1780s) is difficult to conceive of, but you can expand its lighter units and commerce raiding elements, if you are able to beat down British countering ships to a significant enough extent that this becomes more profitable and less risky. Have the French and Spanish win some additional naval battles against the British, such as wiping out some of the British efforts to resupply Gibraltar which could have gone terribly awry, Suffren's campaign go better, the French to press their advantage after battles such as the Battle of Grenada and significantly harm the British fleet. You could also conspire to have British diplomacy be dramatically worse and to cause the League of Armed Neutrality to be directly drawn into the war against the British, which would further spread the Royal Navy out.