Trimaran tour boat with steerable long-tail electric motor ... or what else?

Yodani : Go for it!
Make its roof with foldable or not overhangs all around. Install solar panels on it and you will get a decent amount of power for propulsion/charging, ~ 3 to 4 KW. Panels have not been cheaper than these days, under $1 a watt.
I would use plain automotive batteries, much lower cost and will survive ~4+ years without vibration and proper care. Deep cycle ones are for people who abuse or neglect them and at 3 times the price.

Two motors in a catamaran instead of one on a trimaran can get rid of rudders for low speeds.

 

Automotive cranking batteries are designed to give a large amperage for a short time. The opposite of what you need. A deep cycle battery will discharge a lower rate for a longer time. Also, consider that lead/acid batteries are only efficient to about 60% charge. The effective use is 95% to 60% for long life.

 

Big electric Long Tail