The engine can lock up from "hydro-lock". Water doesn't compress like air does so the pistons will be blocked and unable to move. Most of the time, if the engine was running slowly and not developing much power, no serious damage will be done. Remove the spark plugs, let the starter motor dry out for a day or two, then crank the engine to expel the water. It would be better to do that by jumping the starter relay with the ignition switch turned off, or disable the injectors and spark plugs / ignition coil. Once the engine cranks freely, let it dry out for a few days but crank it once in a while so rust is less likely to form. It will sound funny when cranking because the spark plugs are out and there won't be any compression.
Put the spark plugs back in and try to start the engine. Once it's running, stop it right away and drain the oil to get any water out. If it seems like you drained out more oil than should be in the engine, assume there was a lot of water in it. It might be wise to do a second oil change after running the engine a few minutes. To save a few bucks, it's not necessary to completely fill the oil on the first change. If the engine holds five quarts, drain everything out, then just put in three or four quarts. Fill it completely on the second change after a few minutes.