I'll add a couple of things, having spent the last two winters down there. and having used a chartered Mainship 430 a lot in California.
A week is not a long time. If you are going to the Tortugas, that's essentially three days: 1 out, 1 there, 1 back. So my opinion is, if the weather is good when you first get the boat, go out there immediately.
Ask the charter company now about both recommended AND prohibited cruising grounds, based on how you like to cruise.
The inside, or bay side of the Keys is much more wilderness-like with more gunkholing than the Hawk Channel side, especially between KW and Marathon. I have only explored the bay side west of Marathon on my Whaler, as it is too shallow to comfortably take the 5 foot draft of the Hatt. If the charterer allows the boat inside, find out if the boat will fit under the 20ft new highway bridge at Bahia Honda. The boat we used in CA was 19ft, but double check. A nice cruise would be to go to Bahia Honda from Key West on the inside, anchoring out somewhere in between (lots of choices), then anchor at Bahia Honda and enjoy the park and fantastic oceanside beach, then out out Hawk Channel, anchor in Newfound Harbor, then back to Key West. A fun dinghy ride is to the little "marina" by the bridge between Big Pine and No Name Key, then walk to the No Name Pub for very good pizza and some very "Keys-y" atmosphere.
Another great alternative if you like wilderness boating is to head up to Cape Sable and into Little Shark River and explore the wilds and endless waterways via dinghy from there, then up to Indian Key/ Russell Bay/ Everglades City. This area is all part of Everglades National Park.
Do not assume the boat comes with a dinghy, ask! A dinghy is critical unless all you are going to do is go to marinas, in which case going by car is a better choice. Key West is the only town worth spending extended time in on a short vacation. As for marinas, for a boat that size there are plenty in and around both Key West and Marathon, but not much at all in between. Finding a transient berth shouldn't be a problem. The moorings at Marathon are an alternative. While Marathon has served as our base and is a fun place to settle in for the winter, I wouldn't go out of my way to visit it on a short, one week vacation, though they too have an ok oceanside beach accessible by dink. Expect to pay about $2.50 a foot for dockage in Marathon and more than that in Key West, where we anchor out quite happily west of Fleming Key. Where is your charter boat based?
The best overall guide is Claiborne Young's "Cruising Guide to the Florida Keys". It is very detailed and provides a lot of navigational tips coupled with a lot of on-shore history and tourist info. He also has a good web site, cruisernet, that has thumbnail info on anchorages and marinas, publishes fuel prices, and more importantly has a forum like feature that people post current news and views on.
The Mainship is a good honest boat with a nice layout; is yours 3 staterooms, galley up, or 2 w/galley down? It is easy to do checks in the ER. The master stateroom is very nice for a 43 footer, the bow stateroom is similar to your SeaRay's, maybe a tad bigger, and the middle room is really just bunks, ok for kids. We used one many times all through the California Delta and out the Golden Gate to Half Moon Bay, the Farallons and Drakes Bay, and I can attest to its sea-worthiness if things get rough out there. Still, don't do open water passages to the Tortugas or Little Shark unless the weather is going to be good. The boat will take all but the very worst of it but your crew will not be so content.
Whatever you do, I advise you reserve that boat now; you gonna have some fun no matter what!
George
Hatteras 56MY
Boston Whaler 130 Sport