That would have been 12-14 knots.
“How did you take off? The relative speed during take off would be the same of your ship’s speed away.” That’s challenging!
When the app shows a message about Weak GPS Signal, DJI really means No GPS Signal.
With Apple devices that message can show even when you have a GPS receiver that is enabled.
The main point is to reset during the flight to prevent the drone from trying to RTH to the launch point as the battery gets lower.
At sea I find that resetting to the current location of the aircraft is just as useful for that.
What mode did you set before took off with that speed?