The existence of mosque is not a condition for offering Jumuah salah. Jumuah salah can be offered in field and boat too. However, the availability of Jumuah conditions is necessary. One of the conditions of Jumuah salah is that the place whereJumuah is offered should be a city or suburb or big village which is like a city as per its population and facilities. Similarly jamaat (a group of people) and presence of salah time are other conditions. If the conditions of Jumuah are missing in the boat, then you would offer Zuhar salah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband