Dear Mr/ Ms. Jeff Roberts,
Thank you for your question.
In your case, you should apply a multiple entry visa which will allow you to go into and go out Vietnam many within the time your visa is valid. First, please apply multiple entry visa online at our here https://www.vietnam-evisa.org/apply-visa.html . After you pay for us, we will process your visa letter within 2 working days (for tourist visa) and send it to your email. Then you can print this letter and get a multiple entry visa at Vietnam airport. After that, you can leave by boat and return Vietnam normally with this visa, do not need to apply another one.
Please do not hesitate to contact us if you have any questions.