I seem to remember it taking +10-20m or so for the boat to level out after a crashdive order. For my VIIB crash dive takes me to about 85m then settles out at 70m.
Generally I cruise on the surface at ahead slow; when I receive and alert I hit flank speed and change course rapidly, as soon as I have 10+ knots on the speedo I press 'D' for dive- it normally only takes several seconds for the boat to get below the surface at speed, then I change course again. To recover from the dive press 'A' at your desired depth setting - but be aware that you will continue to dive for about 20m then the boat will level out and return to your set depth.
You can mod the surfaced depth of the boat to improve dive time- my VIIB is set to 7 - 7.5m this can reasult in a slightly reduced surface speed (especially in bad weather) but greatly reduces the time it takes to perform and emergency dive. Sailing with 'decks-awash' is a simple little mod, I'm sure there's a link about this on the SHIII mods sticky.
The faster you are going on the surface, the quicker your dive time will be.
I generally avoid the crash dive button unless I'm in the atlantic- bouncing off the bottom of the north sea after running out of depth is a sure way of letting any persuing DD know where you are!!
On the subject of knowing your sea depth, I think it was Boardinstrumente mod which has a small colour chart attached to the mouse pointer which gives an indication as to the colour depth markings on the NAV map screen- this only seems to work in daylight mode as the colour chart is off when viewed at night- dimmed lights affect the map but not the chart scale.
If you're in the process of being attacked, there's no problem in ordering the NAV officer to ping the sea floor for depth as the DD knows where you are anyway
