damo,
Gearup has given a good summary, but I'd like to add that the reason a VFR pilot might flight fly into the sea in foggy weather is not because he cannot read a VSI or altimeter. In fact, he most certainly should be able to read both if he has passed ANY flight test, including first solo (strictly VFR).
More important in flying in crappy weather is that you can interpret and set attitudes by reference to the attitude indicator/artificial horizon (many different names for similar instruments), and also that you can maintain attitude control of the aircraft for the duration of the flight (perhaps many hours?) with primary reference to that instrument and the power setting. This takes lots of practice and a dedication to disciplined 'scanning' of the instruments, and these techniques should be taught thoroughly during instrument flight training. A good ballpark figure I use is 80% time spent on the PFD, 20% for other instruments and ALL other tasks in the aircraft. This changes somewhat with a good autopilot, but regular scanning still needs to occur.
The absence of good skills in this area will rarely lead to fatality in visual flying but will most certainly lead to serious accident/fatality in IMC (Instrument Meteorological Conditions).
Good luck with your career.
VI