Hey,
Bridges are one of my fav things 
What i suggest is put a ladder into the water so it touches the bottom.
Then they will be able to move around in the water alot easier.
Second figure out what kind of bridge you want… this is important… Plan out the bridge before building anything because it will be hard to sort it out if you make a mistake.
Take a look at my post about my castle for bridge ideas (shameless plug)
http://discourse.stonehearth.net/t/an-island-castle-wip/22469
you will see in mine that i didn’t complete the bridge until i had planned the all the way across