Re: current value of 1982 25hp johnson outboard motor
A 9.9 is worth less than a 25 horse typically - they are much less expensive to buy. A new 4 stroke 9.9 is $1700-$2600. A new 25 horse would be around $4000-$5000. Again - really varies on condition. I don't know the seamist boat or what it is rated for, but as a rule of thumb, you want to have a motor that is at least 70% of the boat's rated power. Boat's are rated for a maximum hp - so if a boat is rated for 100hp, you wouldn't want to have less than a 70 horse on it. When you drop the power below that, you may have problems getting the boat up on plane, which will increase costs and gas consumption.
Having said that, people can and do put motors of less than 70% of the max hp, with varying results. Your best bet is to test both in the water and then go from there.
9.9 sounds low for a 16' Seamist.
www.fibgerglassics.com can tell you what different boats are rated for. Just go to the starcraft page under libraries, look up the year, look up the boat, and it should have the max hp rating.