How big?

Billardiera longiflora is easy to train on structures and will not outgrow the space. However, its fairly slow to fill in. Something you will appreciate in the long run, but it won't get you quick privacy.

Also prone to being damaged by cold winters.

How big? Well up to about eight feet high. Pruning won't be a problem for us.
You know I had it in mind to try grapes. They won't be evergreen or particularly neat looking but........ Or kiwi? I've scanned the posts for kiwis and found out a lot. Will I have a lack of bees, being over the water and 100 feet from shore?
So many questions! ;-)

Thanks for the suggestion. Does this Billardiera longiflora have a common name?
The local parks board has palms growing nearby but frost cannot be ruled out either. How can I find out our zone?

tasmania vine-- it is a bit cold sensitive. not sure when damage occurs, mine sailed through 17 degrees without leaf damage. if you are on the water, should perform for you for many years.

Here is a link that might be useful: some pictures

Also doesn't have any meat on it. The fruits are quite interesting, maybe you could plant it in combination with something more substantial. The color of the fruit would be quite nice with the foliage of purple-leaved wine grape (Vitis vinifera 'Purpurea'). The grapes from this one aren't tops, however.

The simplest thing would be to buy a Fargesia 'Rufa' and plant it in the middle of of a tub. Kept watered and fertilized it would soon make a substantial and dense clump.

I think kiwi is too vigorous to grow in a pot and it isn't evergreen. What about Star Jasmine? The scent is amazing, the flowers very pretty against the dark green leaves. Mine is in a pot and at this point is up to the roof. I wouldn't expect it to freeze if it is on the water and somewhat protected by your home.

Thank you for your suggestions. We plan to be away from the internet for a few days but near some plant nurseries.I'll see what they have....................