They are probably not offering the tour because of the tight time frame. I doubt that you can make it from the ship in 45 minutes with the traffic the way it is in Puerto Rico. You may find an independent outside the ship to take you on the tour but be careful the ship will sail without you if you don't make it back in time.
I agree with Melissa. I think it will take longer than 45 minutes to get there. Coming back, you will be in the middle of rush hour which will probably take even longer. Trust me, I was born & raised in the island and left over 30 years ago. Driving in the island, particularly around the San Juan area, is no picnic these days.
we did rainforest thru rci last year while on explorer I must say we were not at all impressed with the tour or the rainforest for that matter.
basically we saw a nice waterfall but other that that nothing out of the ordinary. I expected to see beautiful floweres and some unusal wild life, neither were present. The guide we had did say that since the previous hurrican had hit the island the forest was not what it used to be but this was in July 2001 and none had it in atleast the past 9 or 10 months, so suite yourself but I think I would pass