Support for worktop - with open shelves so no units for it to sit on

One leg in each corner depending on the length extra legs maybe required

A very quick easy solution measure a kitchen chair or stool work out its supporting weight then transferring these measurements you can assess your worktops requirements

I think a common sense approach to this question is the way to proceed. The factors to bear in mind here are movement , water resistance and the make up of the structure of the boat. There are several methods one could employ to overcome the problem of inadequate support. One could glue a pad of marine ply the lenght of the top by about 200mm deep to the wall to support the back of the worktop. The top would than sit on this and be secured underneath with stainless steel brackets and non rust screws. The front could than be supported by two 75mm chrome legs. Alternatively one could use bespoke oak legs in keeping with the oak top. Oak pads 18mm may be also be recommended under the legs to spread the weight of the top and to protect the floor. The size of these would depend on the condition of the floor and boat. NB I assume the top is 30mm not 300mm as stated in Q. Regards Eugene

well, you haven't mentioned how long the worktop is, only it's thickness and width. I agree with the previous answer, leg on each corner and support legs along depending on the length