Firstly congratulations I hope it all goes well
If it is to happen over a nice meal you could try -
http://www.lemediterranee.com.tn/english/mediterranean.htm - A high end restaurant with nice views over the marina and beach, turn right as you get to the water in the marina through the arch and Le Medditerranee is located at the far end of the marina. Go in a day or two in advance and arrange a nice table etc.
At the far side of the Marina (go round to your left) there is
http://www.sadokkouka.com/LaDaurade/en/index.htm which again is nice and not just as expensive as Le Medditerranee.
There are also some nice restaurants that circle the fountain in the square.
If you really want to push the boat out (excuse the pun
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) you could see about chartering (hire) a yacht / boat for a few hours in the early evening and enjoy a sunset cruise.
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One last thought if your hotel is located on the beach ask if they would set up a romantic candle lit dinner there for you both down on the beach and you can listen to the sea and propose under the stars.
All the best to you both
Graham