I have a boat which is 240v 50 amp and the marina can only get me two 120v 50 amp circuits to me. Can anyone tell me if I can tie two 120v 50amp circuits into one 240v 50amp circuit. It would be greatly appreciated

Simple answer is that two 120V 50A services will give you a 120V 100A service.

There is no simple way to convert a 120V service to 240V.

Saying that, however, your boat's service is probably a 240V/ 120V panel. You should look into the boat beaker panel. It is probably the same for a house. Remove any two pole breakers and use the 120V supply. Most likely the only 240V appliances are the hot water tank and range. They are probably dual fuel anyhow.

To have the main breaker work you will have to jumper the supply side of the conductors together. Each breaker will supply 120V.

The marina's main distribution panel should be 240 or 208. Ask them if you were to pay for the service, to have an electrician convert the circuit over to 240 volts. May be charged a min of 2-4 hours plus materials.

It may be as simple as needing to replace two 1 pole 50 amp breakers to a two pole breaker, and change the twist lock cord cap to a 3 wire 50 amp 125/250 volt configuration NEMA # SS-1.

You may have to sign a waiver to put it back to original condition, but it is worth asking permission to look into, if your willing to front the costs. If there is any competition nearby, they may consider to keep a customer if possible.