In a location where hypothermia is a risk, this is important.
> Approximately 45 minutes elapsed before search-and-rescue (SAR) resources became aware of the capsizing, as the crew did not have time to transmit a distress call before the capsizing, nor did the vessel have a means to automatically send a distress call.
http://www.ussartf.org/cold_water_survival.htm
In this case water was 14C, so while such water may be very unpleasant (depends on overall weather - on a nice summer day it is very nice to go into water at Half Moon Bay where water today for example is 14.5C, and where i originally come from we have 17C, at best, water in summer and everybody enjoys it immensely), it isn't such an imminent danger on its own if one has flotation vest on, as one can survive for several hours in it, yet unfortunately most people wouldn't be really able to actively maintain flotation on their own without the vest, less swim, for duration longer than may be 10 minutes. Basically having vest on or off is the critical juncture.
That's true in this case, but might very well not be the case in a future accident. That rescuers were able to reach the Leviathan II so quickly in this case is partly due to some very good luck after the accident -- crew members spotted a flare just floating in the water and were able to fire it. It was this flare that the first rescue boat saw.