@Renoir 

Hello, for your information, we do have some devices which can connect to an existing wireless network to extend the wifi coverage, including indoor and outdoor devices. 

While the unknown existing network causes an uncertain situation--whether the device would work with it together because the wireless hotspot varies depending on the different countries and manufacturers. 

Besides, to make it work, we need ensure that it can communicate with the hotspot at any time, otherwise other devices may not work once the hotspot has gone. 

And how far away bwteen your boat and the wifi hotspots in the marina? 

Good day.

@Renoir 

Hello, considering the distance from the hotspot, the normal indoor devices cannot satisfy your needs.

We do have some outdoor/business product which can cover the current distance. Like the CPE.

For more information and suggestion, it is suggested to post a thread on https://community.tp-link.com/en/business

Then our colleague will provide you with a specific solution.

Best regards.