How to get people to the fishing island in Cities Skylines

Depends. Are these assets you created yourself or that you downloaded from steam? If the latter, I would say that steam has been plagued with congestion due to the game's nodding popularity it may just take a bit longer after you subscribe. Also, when subscribing, make sure that you have the checkmark on the assets' picture AFTER you back out of its description page on steam. I had an issue where I was hitting subscribe on the description page but wasn't registering immediately when I backed out.

If you're talking about buildings that look like this:

Then you're seeing the European style buildings introduced in version 1.1.0 of the game, released 19th May 2015. You can see the patch notes here

These buildings are what the developers call a map theme (or biome, they used both in the link above), meaning that whether you see the original or European style depends on the map you are playing on and cannot be changed for that map.

You can see this information somewhere on the map selection screen, before you start your city (I'm not in a position to start the game for a screenshot and didn't quickly find an example). I do not think there is a way to change the theme of a map either before starting or once your city is going.