The Mongols had no problem gathering massive fleets of ships from the people they conquered, and were fairly competent at commanding them as well.
If by massive fleets you mean stealing a bunch of river boats and deciding 'it floats in the water, it's the same thing', then sure, they can build massive fleets, however the Mongols had basically zero competence with Naval matters.
Exactly, when was the last time there was a typhoon in the English channel? If there were no kamikaze it's quite likely that the Mongols could have conquered Japan, and England had no military professionals in the same sense as Japan did.
It's really not, as I stated above, the Mongols naval fleet was literally comprised of a bunch of river boats and coastal trawlers, either outright taken or new poorly built ones (half the time sabotaged by the builders) with a tiny core of actual ocean going vessels being commanded by people with little to any actual naval experience or knowledge; had there been no Kamikaze what would most likely have happened is several attempts at landings, leading to fights, alot of ships just sinking after awhile or otherwise becoming useless and the Japanese eventually just realizing that the fleet was to an extent a stationary Shooting Gallery, which would ultimately lead to the Mongols turning back with the remnants of their 'Fleet' and the Japanese losing a few battles but overwhelmingly winning the war.