I have only done the crossings in sailboats (26 to 62 feet) and one trawler. A C Dory is a whole lot different animal. The wave conditions depend on if there is wind against current/or wind with current. As Foggy said--if out of the North--don't go--the seas will be very steep. I would want the wind to be less than 10 knots for any crossing in the C Dory. The boat can handle it, but it will be very comfortable. The "best" time is in the summer, when there are much less chances of anything out of the North. (wind against the gulf stream).
I like the Windfinder Pro app--mostly for sailors, kite boarders and wind surfers, but have the best wind predictions (and sea state, etc) for this area. Also you want to check with Bahamas weather as well as the NOAA weather.
Always know what the weather is at your destination, as well as the origin and in route. For example the current wind is about 17 knots with gusts to 22 at Miami and only about 6 knots at Bimini…but the 17 knots out of NE would be enough to be a "no go" for me.
Another great source of information is the Waterway Radio and Cruising Club.
http://www.waterwayradio.net
The broadcast starts at 7:45 AM ET, 7268 Lower Side band. Many of the good radios will pick this up. I have one of the Grundig Yacht Boy 400's which I always carry, even if I have my ham radio aboard. (replaced by the Grundig G4000A and more recently G5. other receivers include: SONY ICF-SW7600GR:
see:http://www.swling.com/Radios.htm
As a ham, you can check in (and I would encourage joining the net for only a few bucks a year.) You will also get the "latest news" from the Bahamas. You don't have to be a ham to listen.
Are you going to trailer the boat to the launching point--and where are you gong to stow the trailer. You want a secure facility. Also you want to be sure that your boat is secured well when in the Bahamas. A lot of reports of theft. But a lot depends on where you are. Staying on the boat, you are far less likely to have a problem.
The shortest distance is West Palm to West End Grand Bahama--about 60 miles. An option is to go West Palm area to Freeport Grand Bahama (about 70 miles), and check in. That way you have your leisure to work down to Nassau, and not have a long run if you have bad weather. This also allows you to pick up fuel in case this is an issue. It is another 120 miles down to Nassau.
Garmin blue chart mobile also have wind arrows, but not the prediction.
The Gulf Stream velocity is 2.5 (rare) to 6 MPH, so that has to be taken into adjustment for your calculations of course.
I cannot comment on the check in/check out--because it has been some time, and things change a lot thru the years.
Life raft: I am one of the nuts who has never carried a life raft, except when required for offshore ocean races. I have carried a high quality hypalon dinghy with manual CO2 bottles fitted to the inflation valves and a custom made canopy for the inflatable, which lashes to the rope on the sides….a hand operated water maker, water, supplies, a really good ditch bag etc.
If there is any question--get a life raft--one of the coastal platforms is far better than nothing.
Essential--an EPIRB or at the least a Personal Locator beacon. I carry an EPRIB and each person has their own PLB, and a waterproof hand held VHF radio, to carry on their person!
Satellite phone--can be rented. I have not used one, but I have had ham radio on the boats in open water passages like this.
Sea Anchor--again, not something I have owned, but again, my voyaging has been in large sailboats, and I had what is called the Jordon Series Drogue. If you want a sea anchor, then consider the Para Tech--probably the 9 foot size would be best--but you need swivel, trip line, chain, chafing gear etc.
In summary, this is not a passage to be taken lightly. It can be very rough, or as smooth as a mill pond. It can change very rapidly. IN the summer you see PWC and even open CC going over…But it is serious water.