For marine SSB, there are only two licenses, not three. These are:
1. the ships station license; and
2. the operator's license (either restricted or higher).
The ships license is the same one used for all onboard transmitting equipment when you go foreign, or for commercial use.
The operators license comes in different flavors, from the lowest restricted license to the general license (GROL). There are several varieties, but for most people its just the Restricted License which is needed.
Yes, you can use a marine radio on the ham bands IF you have the proper ham license. The reverse is not true, however: you cannot legally use a ham radio on any band other than the ham bands even if you hold licenses for other services (marine, aircraft, etc.). Radios are "type-accepted" by the FCC for specific usage, and no ham radio is type-accepted for marine use.
The use of a non-ham radio (e.g., marine, commercial, military) on the ham bands is generally accepted practice because a licensed ham operator can build his own radio or modify other radios to work on the ham bands.
Note, however, that the reverse is not necessarily true. If you modify a type-accepted radio in most any way, it will nullify the type-acceptance for use on the originally intended band.
You can use a ham radio or any other radio to receive anywhere -- at least in the U.S.. But, you cannot use it to transmit anywhere except on the portions of the ham bands for which you hold a valid license.
What does this mean, in practice?
1. Anyone, licensed or not, can purchase and install a ham transceiver and can use it to listen on any band/frequency desired. This includes ham, marine, aircraft, weather fax, etc., etc.
2. You cannot use this radio to transmit, however, except on the portions of the ham bands for which you are licensed.
3. In an extreme emergency you can use any means to attract attention and call for help. You might have to fight with the FCC or other authority later on, but if it's a bonafide emergency and you have no other legitimate means of getting help, you'll likely be in a good position.