Engine compartment flooded!

Im borrowing a mates 2009 Four Winns 222 SL in 2 weeks to take over to the Isle of Wight festival. We went down to the boat today to do some bits. The boat is on blocks ashore and is due to go back on its dock next week. When we got there I opened the ski locker and it was full of water! Looked in the engine compartment and the bottom 1/3 of the engine was under water!

The yard staff are supposed to undo the bung when they store boats ashore for the winter but for whatever reason they didnt.

We have had some heavy rain and as the bow ladder locker drains into the bilges the batterys must have gone flat as the bilge pump would have been constantly running. Both batterys have now bwwn put on charge as they were totally dead.

The engine is a 5.7 volvo 5.7 (320hp) and as the lower 30% of the engine was submerged the starter motor was totally underwater. 'Luckly' the ecu and control boxes were still dry.

So where do we go from here? We have pulled the bung and drained out all the water. The batterys are on charge. Do we just go right ahead and try to fire it up or is there any preventative measures we should take first?