This image from google maps shows the Solent, a stretch of water separating the Isle of Wight from the mainland. The area has two major docks, one up Southampton water to the top left of the picture and Portsmouth with it's naval dockyard visible to the right.
Not to forget the oil terminal at Fawley in Southampton water, from where fairly large tankers like this one come and go on a daily basis.
This overlay map from shows the AIS tracks of shipping in a single afternoon. giving some idea of the density of traffic.
To make matters worse, the deep water channel takes large shipping in a big loop around the Bramble Bank , north of Cowes, before heading up into Southampton water. In this area ships have a moving exclusion zone 1000 meters in front and 100 meters either side, where small craft are fobidden.
Even in these difficult conditions these big ships move fast and can come up on a small yacht very quickly. Southampton Harbour Master has an excellent guide for yachtsmen on the VTS web site - the old myth about power giving way to sail does not apply here.