An air carrier crew departed RSW and was vectored west for weather. ATC vectors took the aircraft more than 50 nm off shore so the crew declared an emergency in order to return to a filed overland route.

Narrative

Departing RSW we were originally filed the CSHEL1 departure to ZZZ. Clearance was changed to radar vectors to HILTI SZW MGM as filed. After departure assigned westerly heading by MIA Center. Level at 230 FT and heading west for considerable time. We asked for higher and a turn; concerned over the 50 mile limit. We were given a frequency change. On the new frequency we asked for higher and a turn. We were told to return to previous frequency. On the previous frequency we were given higher and direct HILTI. Due to thunderstorms over and west of HILTI we could not at this point go to HILTI. The quickest way back towards shore and avoiding the storms was to deviate north and west of HILTI which left us outside of 50 for the extent of the weather deviation. An emergency was declared over Center frequency. MIA Center was asked for the quickest way around the weather and back on course and they agreed with us that direct SQS when able would be best. The flight was completed without further incident.