I can't tell you anything about Mears, but as of 2007 taking a taxi was not hideously expensive for 2 people. Round trip was about $25 including tips (this was from POP to DC), but I really don't know if rates have changed. If you're on property, you can ask a hotel CM to tell you which taxi services are reputable, and I feel like I've heard they'll only let certain services on property so no one has to deal with anything shady, but I could be wrong.
I'm going to base this on staying on property because that's what I have the most experience with.
I figure it this way...
Go to Discovery Cove, stay til 5 or 6 ( because I think that's as late as its open) then:
Use their shower facilities and change into whatever you want to wear to 'Ohana there, take a taxi back to your hotel then use the Disney buses or what have you to get to the Poly. If you do it this way, you and your boyfriend can shower at the same time, and even if it takes you an hour (which it won't) to get back to your hotel, that puts you at 7 at the latest, then figure in at the most you'll need an hour to get to the Poly. That only puts you at 8.
Use their shower facilities to rinse off, take the taxi back to hotel, take a full shower there, then use the Disney buses etc etc to get to the Poly. If you take this route, you can rinse sand salt and sunscreen off at DC, take the taxi back to the hotel so let's figure half an hour there, which would be 6:30. You can take seperate showers (or together

) and even if you take an hour and a half between the two of you to get ready, that'll put you at 8, then that safety hour of Disney travel, you should be at the Poly by 9 at the latest.
Hope that helps some!