Are you sure the surface tablets have GPS? Everything I can find about them on the Microsoft spec pages only lists digital compass and accelerometer, not actual GPS. You could get a surface pro (traditional intel x86 based instead of arm based surface) and get a USB GPS dongle to use OpenCPN, but that's a pricier option.
Now that I look closer, maybe it doesn't.
I think the old RT version did, but they nixed it in the Windows 8 version. And RT looks to be being rejected by consumers.
Oddly, I've found that many tablets don't have GPS. Maybe the chartplotter mfgs are paying them off
I did find a Toshiba WT8-64 with windows 8, MS office (need) and GPS. I can add a keyboard, it has video out to connect to the bigger screen on the boat for downloaded movies, and microSD and USB for storage and backup. It's about the only one I can find that has all this. Now if I can find the right 12v adapter (another item oddly missing from many tablet mfgs) then it might be golden.