It really depends upon exactly WHICH lenses you have.

Canon has made four different 70-200mm f/2.8L lenses... the first two DO NOT work well with teleconverters. Those are the non-IS version and the original one with IS. The 70-200/2.8 "II" and "III" both work pretty well with Canon EF 1.4X II or III. They are also pretty good with the 2X II and III TCs.

The original Canon 100-400mm f/4.5-5.6L "push/pull zoom" works pretty well with a Canon 1.4X II or III, but not with any 2X (and you'd have no autofocus with a 2X on this lens).

The current 100-400mm f/4.5-5.6L "II" works very well with Canon EF 1.4X II or III. There is very little loss of IQ to the 1.4X. I have no idea how good image quality is with any 2X (also would mean no AF).

There's not much difference optically between the II series and III series teleconverters. There may be some difference in AF performance (when Canon first introduced the III series they actually said the TC electronics slowed AF a little with some lenses, for better accuracy... but I've never had any means or opportunity to compare the two series AF performance).

This will get you to 560mm f/8 equivalent with good to great image quality.

If it were me, I'd consider getting (renting or buying) a second 5DIV, for the better AF system and higher resolution. With 30MP (instead of 22MP), you would have more flexibility to crop a little, if needed. For one, I don't like to go to a job without a backup camera (heck, I'd probably have the 5DIII in my bag, too). I'd put the 70-200mm on the 2nd camera and have it ready to shoot at a moment's notice. In fact, this is exactly how I shoot sporting events.... two cameras... one with a longer telephoto, the other with a shorter one. 90% of shots can be made with those. I also make a point of the camera's being identical models, so there is no delay switching back and forth between them. When shooting fast moving events a seemingly minor difference between cameras can make for a pause that misses a shot or cause a mistaken setting.

I'd also recommend a monopod (I usually use a tripod, but I'm not on a boat). The 100-400mm isn't terribly heavy for short periods of time, but after a few hours can feel like a boat anchor!

Another lens that's always in my bag and ready to swap onto one of my cameras for closer work is a 24-70mm f/2.8. You might also want to include an even wider lens (I carry a 20mm f/2.8 when shooting full frame like you, or a 10-22mm when shooting APS-C).

Shooting from a boat out on the ocean is one of those rare instances when I would use UV/clear protective filters on my lenses. The filter not only helps improve the lens' water resistance, in case of a splash, it also keeps "salt air" off the front element. In case you don't know, that stuff can be nasty to clean off and I'd rather scrub a filter than the front element of my lens (it's barrel and the camera will need wiping down, regardless). I'd also take my Circular Polarizers!

One more thing... if you shoot with two (or more) cameras, be sure to sync their clocks the day before or the day of the event. It's quick and easy... just use the USB cable and hook each of them up to your computer and use Canon's Utility, navigate to the clock and click the "sync with computer" button. Done. This way they'll be identical and the mix of images from different cameras you come back with will be in correct sequence. The reason this is needed is because different camera's clocks lose or gain time at different rates. Even after a few days they can be several seconds different. If you haven't synced them in weeks, months or maybe forever, they will be quite far apart and your images will be way, way out of sequence. Even a few seconds difference makes a mess of things (been there, done that when I've forgotten to sync my cameras).

For several years I wrote and photographed for Noreaster magazine on Chesapeake Bay. I only did it once, but I did shoot races out of Annapolis and had results I’m very proud of. My advice is for you to just trust the committee boat. They will stay close to the racers but far enough away to ensure that they don’t interfere with the race.

For gear, if it’s sharp enough, use your longest zoom. Leave the rest of your gear at home.

Shoot at the lowest ISO the weather permits, set your shutter speed around 1/500, and use somewhere around f/8-11 for your aperture. DOF was always my least worry, but I was generally interested in one boat at a time.

Good luck and don’t forget to bring a water bottle.

Not mentioned is that with your Canon 5 iv, you can take videos. If you can get in position, get a video of the boats rounding a mark and setting "Spins Acre". Whether they haul it up or unferl it by breaking rubber bands there is a short time (before the helm and guys on its sheets gets it under control) when wierd things can (& often do) happen. Super video opportunity.....Tacking with a main & geny is a disaplined orderly operation, not so (ever) with a spinnacker...