Receiving "USB devices disabled: Unplug the device using too much power" upon login. Built-in keyboard and trackpad sometimes become unresponsive
If there has been water damage that will have dried by now so the likelihood of water sloshing around causing this issue is near 0, however it may have caused corrosion to the trackpad, keyboard or main logic board.

You need to get Apple, or an Apple Authorised Service Provider to repair it. Alternatively you could seek third party service, or do the repair yourself.

The USB Over Current warning is associated primarily with the positive and negative rails having been short circuited (connected) causing more current than the allowed to flow, the USB chip puts that port into a disabled state to protect the computer from damage, this could be the result of corrosion inside creating particles that are shorting a component, or it could be that one of the connectors or wires has become corroded and has increased the resistance of that wire again causing a higher current draw and triggering the protective mode.

Self repair

If you decide to repair the computer yourself, you need to think about where the water was spilled, and how much, if it was only a small amount on the keyboard that could suggest that only keyboard replacement is necessary.

Anyway, first step is to backup data, and find a suitable take apart guide from a site such as iFixit, following these guides you can open the machine and look for signs of liquid damage on the keyboard, trackpad and main logic board.

Once you've identified where the liquid has damaged you can proceed to replace these components.