Sure, that sounds like an exciting trip! Here are some useful travel tips for your visit to Miami, Nassau, and Havana:

Best Time to Visit:
The best time to visit these places is from March through May since the weather is pleasant and it's less crowded. Though, Miami can be visited virtually any time of the year.

Route and Accessibility:
You can start your trip in Miami, then take a flight or cruise to Nassau, and from there continue to Havana. There are multiple airlines and cruises servicing these routes.

Altitude and Acclimatization:
There are no significant altitude concerns in these locations.

Accommodation and Booking:
It is advisable to book your accommodations and flights in advance, especially during peak tourism seasons. There are a variety of options ranging from luxury resorts to budget hotels.

Clothing and Essentials:
Opt for lightweight, breathable clothing, swimsuits, sunscreen, and a hat. It's also a good idea to pack a rain jacket, as tropical showers can occur unexpectedly.

Health and Safety:
Stay hydrated, especially when spending time outside. Travel insurance is recommended. For Havana, prior research on local laws & customs is advisable.

Local Etiquette and Respect:
In all these places, it's important to respect local customs and traditions. In Havana, a basic understanding of Spanish can be very useful.

Please note:
There are special requirements for traveling to Cuba from the United States, including specific reasons for travel. Be sure to double-check these requirements before planning your trip.