Skype is free but will use an amount of your mobile download data. If you have unlimited download, unlikely, then this isn't a problem but may create problems. You will also need to make sure that where you berth/moor then there is adequate mobile coverage. As far as I am concerned a mobile phoen tethered to laptop or other device is the same as mobile dongle. Same problems with data usage/costs apply. Assume this is UK only as any roaming would push costs extortionately high.
OnePlus 5 review