You'll be all right, the airport's undercover and the bus from the airport to the Sheraton will have a roof on it. The floods happen every year so they are used to it.
Be more worried about a bumpy ride coming in and out of the airport.
Lagos is a great place, enjoy. Nothing to worry about so have fun, look forward to seeing your report when you get back.
None of the above photos were taken by me, they were on a disc I found in a dustbin............