When the winds are blowing out of the North, against the direction of the flow of the Gulf Stream, it is very easy to tell when you have reached its magical waters. The waves will build high and break in steep fashion and the crossing will be miserable!
If the winds are blowing from the South, with the direction of the flow of the Gulf Stream, it will be much less noticeable. When the wind and current is going in the same direction, the Gulf Stream can be rather hard to denote.
One of the biggest ways to tell that you are in its waters is your position will not match up with your dead reconing because the current will cause you to drift north as you move east. On a GPS display, you will note a deviation between the heading of the yacht and your course over ground. As you are pointing East, yet moving slightly North-East.
This is all well and fine, but it takes navigational equipment to identify the stream. What about a method that may seem out of place on a cruising yacht? What about water temperature?
The waters along the east coast of the United States will usually be in the range of 60F to 75F. The Gulf Stream on the other hand will typically be well above 80F. By measuring the water temperature, you can easily discern when you have actually entered the stream and also tell when you have exited the other side of the stream.
Having a salt water plumbing setup makes this job much easier, as you can simply fill a glass with sea water in the galley and let the thermometer do its thing! If you do not have the luxury of salt water plumbing, you could also pull up a bucket of sea water every now and again to measure the temperature of the water you are passing through.
I would caution against towing a thermometer behind you for a few reasons. First, a fish might find it to be an intriguing morsel and it might get bitten off its tow line. Second, the tow line could also break. Both of these scenarios involve you losing your thermometer, in which case you would no longer be able to accurately measure the water temperature. The other problem with losing your thermometer is they tend to contain plastic components which would break down and add to the already significant plastic pollution problem in the oceans.
For these reasons, I would highly recommend that you pull sea water into your boat and measure the water temperature in the safety and security of your yacht.
The Gulf Stream can be a tame river in the ocean that you innocuously pass over or a nightmare, all depending on the winds and how they interact with the current. Be sure to cross the Gulf Stream only when the wind is blowing with the current. Otherwise, as others have told me "Add 20 knots to whatever the forecasted wind speed is, that's what it will be like out there."