Where do you place boat house?

You place it the same as all the other decorations, on the island wherever there is enough empty space, i.e. 2x2 or four squares. You can then proceed to build a lake or river around it using other decorations such as 'stream', 'lovely lily pads', 'lush reeds', 'merrily rowing', 'romantic swan boat', 'swan boat station', 'duck dwelling' etc. Once in a while you will also get seasonal decorations that can be added on to expand your water body.

You cannot place it in the sea.

Edge of a lake seems to make sense. Use multiple stream tiles together to form a lake. Know that you do not have to buy everything, just pick what you like. If you need more decorating ideas, join "June's Journey Orchid Shores: Island Decoration - Official Wooga Group" on Facebook.

I like the spot down low near the docks/pier.