The entrance to Marina Bay is just S of the airfield. Entering in a strong SW'ly it's probably real difficult but will get better once inside the protection of the North Mole. Keep the 3 fuel pontoons to your right. VHF Ch 71, after contacting they'll tell you to change to another channel and then will direct you straight to a berth where a marinero will assist to tie up. The berths to Marina Bay are to the left of the office, a white two storey building. The reception pontoon is to the right of the office, but is often in use by the real big boats.
Moorings bow/stern-to with mooring lines and concrete quays. The tide is significant and causes occasional big steps up or down to your boat. In SW'ly winds the berths are not very comfortable and also with all the movements of ships in the bay it's never really calm.
Getting there and away takes some tidal planning. It pays to get hold of a tidal atlas.
F.i. leaving westwards you should start at 3 hours after HW and going eastwards at HW minus 3 hours.
Free wifi from the marina, but it often didn't work. Several bars provide wifi, which you might be able to pick up from the boat. Our Spanish dongle worked well on a Spanish signal and also our mobile telephones displayed a Spanish provider.
There is a laundry place just behind the front line of bars, and it also has big self-service machines at ₤5.
For shopping there is a huge Morrisons , a couple of smaller supermarkets, Ramsons and Coviran, along the road to the cruisers terminal (all also open on Sundays) and a fresh food market on Market Square. The office will give you a map.
There are three chandlers: Sheppard's and Marine Maintenance, behind the bars and restaurants, and Hire-U-Store just past supermarket Ramsons.
The shipyard is not really suitable for yachts. We heard the sailing school takes its boats to Smir in Morocco, where there is a yard run by an English speaking Spaniard. (We've used it, in 2009, and it was good, but you have to take all your paint etc. along yourself.)
There is a vet on the second floor in the Shopping Centre building inside the Casemates Square.
Gibraltar is tax-free and cigarettes, spirits and fuel are cheap, but other things are not really cheaper than Spain, and you better stock up with beer and wine in Spain where it is half the price from Gibraltar.
Suitable for overwintering? Several do, and we ourselves like the place so much that we would also like to, but we don't want to put up with a lot of rain, wind, fog, cooler temperatures than other places some 100 miles east or west, and downright dangerous situations in storms. But then, if you like Gibraltar a lot, or might have found work here, you might not mind all that. Security not great with free access to the pontoons.
Gibraltar has free local buses, which are great for sightseeing (don't miss The Rock), but at the “station” in Market Square you won't find any bus routes or timetables. To get further into Spain you need to cross the border and go to the bus station in La Linea. After crossing the border turn left along the coast road and then the first street to the right. As there are several bus companies operating from La Linea, the best bet is to check at the bus station itself.
On the road map it looks like there is a railway station in La Linea, but there isn't and the easiest option is to take to bus to Algeciras and get on the train from the station next to the bus terminal. The website for the trains is http://www.renfe.com/, a bit of a maze. I happened to come across an easier one: http://www.bahn.com/i/view/GBR/en/index.shtml, but that one doesn't give prices and they can vary a lot from one train type to the next. Return tickets are much cheaper than 2 single fares and I've understood they are valid for 2 weeks.
And of course Gibraltar has an airfield with flights to the UK.
Prices reasonable to cheap, but the winter rates are only for 4(!) months. Water and electricity are metered, with water being quite expensive at ₤ 0.01 per litre, but electricity cheap at ₤ 0.15 per kWh. But their showers are free with plenty hot water. http://www.marinabay.gi/fees.html
The two other marinas in Gibraltar are:
The new marina Ocean Village, located where Sheppard's Marina used to be. Their Premium Berths are ₤ 1 more expensive than Marina Bay. For that you get floating pontoons, so no variable step up/down your boat, plus a floating finger pontoon, however without cleats, free satellite television, a red carpet (true) and the pontoons are only accessible by electronic cards, but the office is the same and the showers/toilets as well (under the office), so you have to walk further. In bad weather the berths are possibly not really more comfortable than in Marina Bay. http://www.gibraltarberths.com/prices.html
Queensway Quay, located a bit further south. It has floating pontoons, but the berths are sideways onto the incoming swell. If you like a marina without the noise of the airfield, surrounded by classy restaurants and luxury apartment blocks, that's the place to go, and it's even cheaper than the other two. However, reportedly, safety-wise it's the worst from the three in winter storms. http://queenswayquay.com/about.html
And if you'd like to compare them with the new marina Alcaidesa in La Linea: http://eng.puertodeportivoalcaidesa.es/tarifas/