I've used Eagletree loggers. Chiefly for current draw and measuring RPM. The ability to measure these aspects with the vehicle whilst under way is an absolute boon of modern microprocessor technology.
I do have a plug-in GPS module, which can measure speed, but haven't used that part on a sub, only in aeroplanes, cars and boats. Sadly the signal won't penetrate through water, which would mean surface running on, or fixing it on a separate pontoon.
You can always measure speed the old fashioned way with a stop watch timing over a measured section of pond/lake/pool. Accurate enough for me.