All of the above.
As mentioned, try and find the cable coming into the boat from the shoreline connector (unless the connector is already in the boat as is sometimes the case). The insertion point of the isolator should, ideally, be as close to the shore connector as possible. However, as long as the isolator is anywhere inline between the input from the shore connector and the cables first point of contact/connection in the boat, then this will be fine (i.e. the earth coming into the boat must not have been connected to anything else before the isolator). The only slightly awkward bit is using a nice sharp Stanley or craft knife to carefully split the outer cable insulation to expose enough cable to allow the earth to be shortened (without actually cutting into the earth/live/neutral wires!) and then connected to either end/terminal of the isolator. The remaining live and neutral wires can then be neatly wrapped in insulating tape to protect from chaffing etc.