Well, you have to know that there are quite millions of people out there, such as the Bajau tribe, who actually born and live their whole life nomadic on their homeboat, only once in a while landing for trade.
And about the lawn, it brought up since Robin has become true mugiwara crew, to be able to relax at last and to fulfil her hobby on gardening and planting her lovely flowers.
The lawn is also brought up since recently Japan is working hard on developing the highly-advanced greening system in urban centers, high-raised buildings, etc. (see the biolung or mitsubishi miraikan etc in aichi expo) so now it is not strange anymore to be able to create a lawn up a ship deck.