If you are lucky and have connections upstairs, the rainwater has only been there for 2 days and you are good to go.
More likely severeal years and you will have internal corrosion
Seen the same on a project
boat I had here in Fort Lauderdale
: no drains, no bilgepumps, hull
filled up with water, leaves from trees above fell in and rotted. slowly she turned into a garden.
Not one thing could be used again including the engine
(The boat had been sitting for 10 years)