I would install a conduit as well. And the wire you linked to is expensive. I pay about $30 Canadian for 14/2 tinned duplex (100' roll) in Victoria and that is $38 for 18/2. In the US I'd recommend GenuineDealz for a good price. Not sure if I would use single strand or duplex. But I would certainly use a heavier gauge than 18. Maybe 12 awg. And use a separate ground for each light, not a common one.
Living aboard in Victoria Harbour