I've used umbrellas in the past and broke two of them. I had one of the stronger patio umbrellas from my outdoor set and then the Costco special one. Both snapped in the Deso wind. Umbrellas suck! Hard to handle, don't provide much shade and a bitch in the wind.
Go bimini all the way! I first got the 6 footer and it was too short. I got the 8 footer now and it's just enough to cover me, my wife and kids on the front bay. If I was getting another, I would definitively go 10 foot (on a 16 foot boat).
The bimini provides MUCH more shade than umbrella, you can control the shade and angle much easier. I actually rowed Deso wind with bimini up and was able to move forward.
It's a great cover for the rain as well! And the coolest thing about bimini is the sail! You can sail with down river wind in the morning by moving the bimini forward and holding the back lashes.
Also it's very easy to fold over the gear pile in the stern.
I got mine off some online boat web site. Just google, you will find the right one. Get the right width and go 10' if you can for 16' boat, you won't regret!
This is my old 6' bimini:
As you can see it barely provides shade for one passenger bay:
Example of umbrella coverage... not much and a small gust of wind will snap those umbrellas:
This is my 8 foot bimini, you can see that it fully covers passengers and gives some shade for the captain: