Ive played with GPs trackers a lot and have fitted over 20 so far in RIBS two of which have already been used to recover stolen RIBS
If you can modify your unit as follows, I will start buying them from you.
1:- make it waterproof
2:- put the aerial in the case and use a patch aerial ( they are fine on every boat I have fitted them to)
3:- make the unit look like anything other than a gps tracker ( so remove the vanity branding as these work best if no one knows they are fitted)
4:- a waterproof panic button so send emergency message to distribution list, I know we all have VHF but all help is good.
5:-remote shut down ( programmable normally open or nomally closed dry contacts to put in kill cord cct)
6:- flashing light on Dash that can be activated by sms (trust me I have this on current unit I use and its really useful )
7:-NMEA 2000 /183 GPS output because if you have another GPS on board it really makes sence to be able to use as a backup if its needed especially useful if its nmea2000 as any other display on network can use the data as a backup to your primary GPS.
8:- have a customisable SMS response message format that can insert the position into a url that you can click on a mobile to show position in googlemaps., i.e. no more cut and paste location info.
Good Luck as I think a reliable afordable unit will sell well.