Icelanders are invariably helpful, but be aware that there are no facilities for yacht repairs (no travelift anywhere in Iceland) and although you might find a diesel engineer,he will not have any yacht spares and no means of obtaining them in a hurry.
Yachts are welcome in all Icelandic harbours and very few ever ask for any berthing charges. In the summer months when the deep sea trawlers are away, they are often quiet. There seems to be no need to radio ahead. Choose a sensible berth away from shoreside activity and it is unlikely the harbourmaster will ask you to move.
The harbour walls are always clad in tractor tyres and a fender board is essential.
Presumably because of the freezing winters, fresh water is generally not available by tap. Instead, look out for a stout hose which will be running into the sea from the top of the harbour wall and full up from that.
Every small boat harbour will have a diesel pump but these are only available to local fishermen who have a card key and an account with the diesel supplier. However, I used a Mastercard in one of the diesel pumps beneath the cafe in Hafnafjödur.
The Brokey Sailing Club (Reykjavic) give directions for obtaining diesel and these may apply throughout Iceland.
Call +354 5509933 between 0700-1700 on a working day to make an appointment for delivery by tanker. Some reports say that this will be at pump prices, others say you will pay a premium.
Diesel was obtained easily be us at Isafjödur, Grimsey and Nordfjödur. The filling station at Neskaupstadir has a small jetty and diesel pump but no useable at low water.
Cash is hardly needed. Cards are accepted even for the smallest amounts.