Water temperatures range from about 4 Celsius in January to March, to about 13 C in July to September. A dry suit with a warm undersuit is needed for most of the year. A thick semi-dry may be adequate from late spring to early autumn.
Tidal currents can be very fast indeed. It is essential to consult tide tables before diving.
Visibility can be very variable. The west coast of Scotland has a reputation for better visibility than the east. However, I have experienced visibility up to 15 metres and more in the Moray Firth (also 1m and less!).
The tidal currents around Scotland can be wicked. You will need to know about the tides if you intend diving anywhere. and the NTSLF web site provieds tidal predictions for UK and Irish ports.