Guernsey has an excellent bus network with regular, and reasonably-priced, services covering the whole island. During the day, certainly in summer, most routes run with a frequency of at least 3 buses an hour and there are a variety of ticket options available including an all-day pass.
The bus depot is situated in St Peter Port, a few hundred yards south of the harbour from which most of the services originate and terminate.
The main tourist route is covered by the #7 and #7A service which pretty much circumnavigate the island in respectively clockwise and anticlockwise directions and these stop at, or close to, most of the attractions, the airport, several of the smaller towns and almost all of the beaches.
The Tourist Office's freebie map shows all the routes and also stocks up-to-date timetables.
The main ferry operator between St Peter Port and the UK is the Condor Ferries company. They offer regular fast ferries to and from Weymouth and Poole and a daily conventional ferry service to and from Portsmouth.
Ferries from Weymouth take just over two hours whilst those from Poole about three. For those in less of a hurry the Portsmouth to Guernsey conventional ferry takes about seven hours going and thirteen hours overnight to return, sailing via Jersey.
The fast ferries are a bit cramped and whilst they do have a bar, cafe and a duty free shop they aren't as comfortable as the big ferry and so if you are out to enjoy your trip then the Portsmouth run is advised.
Website has all the info and online booking facility.
You can take a bus around the entire island, it takes about an hour and a half and cost 50 pence. Bus no. 7 runs clockwise and no 7A runs counter-clockwise around the island. Pick up attractions from tourist information center.