Absolutely... have a spare, and the tools to change it. I know some people get crazy with carrying an extra hub also... but in all my years fo towing trailers... I've never lost a hub. Basically, the way I see it... if you are looking at your trailer once in a while, and checking things... you will know if you have bearings going bad. Heck... I towed my old sportster home (when I bought it) from Ohio back to Pittsburgh, and it didn't even have grease caps on the hubs. I'm not saying you should try that... but bearings don't just spontaneous explode.
In my younger, dumber days... I had a little motorcycle trailer, and I never had an extra tire. AND... 2 times... I had a flat. One time I was lucky. We were right by a wal-mart, and got a tire. The other time... we were way out in the desert, and I spent 3 hrs driving back to town, and finding a place that had a tire. (Lost half a day that I could have been riding) Since that trailer didn't have much weight... it wasn't a big deal when it went flat... but with the weight of a boat... it could get dangerous if the tire blows.
As far as the pressure... it says right on the side. If you are doing a long trip... run them at max pressure. It will help with MPG, and it will also help keep the tire cool.