The same laws apply to boats as apply to vehicles.
If you have a handgun carry permit, definitely. The handgun carry permit covers "any handgun that you legally own or posses."
You can not carry an unconcealed handgun in Texas with or without a license.
It isn't generally illegal to carry on a boat, assuming you can otherwise legally carry a handgun, however, it may be illegal in certain locations. If you are on a federally controlled waterway, it is probably illegal, or some states may have restrictions.Added: Check the laws of your particular state. The same law applies to boats as would apply to motor vehicles on state waterways. See above answer for the answer to federally controlled waters.
No
All countries - once you have legal permission, allow you to carry a handgun.
No
It depends on what you mean exactly. You can transport an unloaded handgun in your vehicle while you travel as long as the handgun isn't accessible to you. However, if you have a handgun carry permit, you can carry the gun on your person in some states. That said, you can carry a handgun on your person inside your car in some states without a permit. Check out handgunlaw.us for more information about each individual state's laws.
You can legally transport it unloaded and inaccessible, but to have it loaded and accessible, you must have a NY handgun carry permit.
No, Washington DC does not allow concealed handgun carry, nor do they recognize permits from any other state.
Arizona is an open carry state. If you can legally own a handgun, you can carry it openly without a permit. If you want to carry a concealed handgun, you need a permit.
YES
It's unlawful to openly carry a handgun in the state of SC. A bill has been recently introduced to openly carry without a permit.