Midbow. Know everything the bowman has to do and look for opportunities to back him up when he needs it. Get halyards off the mast and to him, run sheets if they need it, grab the jib bag and help flake the back end. Etc.
Sewer: pack chutes. Fast. Pack it into the bag in the reverse of how it'll come out. Foot and belly in from the bottom up and then 8 feet of the clew and tack at either sides of the bag. Learn how to run tapes efficiently. Don't ever disconnect anything without a good reason.
Floater changes from boat to boat but basically you need to do a variety of things that have to happen when there aren't hands to do it.
As left hook stated but also could include the following depending on the size of the boat e.g modern IRC 45 to TP52 or bigger:
1. Mid bow - if the bowguy is out on the sprit, pole, up the mast, at the kite clew changing gear, then bow is run and maintained by the mid bow until he is back in position.
2. Sewer may be also be a mid bow position in buoy racing but owns the sail stack below (especially distance racing when you are carrying staysails, MGs, code-0, etc), packs kites, clears the sail after string drops, fixes sails if needed, runs zippers and properly rolls sail turtles for the sails that are up.
3. Floater - depending on the boat , you may need two pit people to handle halyard locks, even if the halyards are brought up on pedestal run winches (no mast man), floater may be the second pit person, the sewer and assist with sail drops