Visitors from the US, Canada, Australia, New Zealand, and many other countries, as well as the EU do not need a visa to enter the Channel Islands (or the UK). The UK Border Agency website, http://www.ukba.homeoffice.gov.uk/visas-immigration/visiting/, provides further information on visas. Guernsey is free from exotic diseases and vaccinations are not necessary.
Health and Safety
There is little to be concerned about on Guernsey. In fact, it is safer than most places in the UK proper and in France. Local folks are friendly, crime levels are low, and the weather is not extreme in summer or winter. Pickpockets do operate in St Peter Port, though the activity is far from rife, so remain as vigilant as you would at home. Avoid dimly-lit city streets at night and make it a point to withdraw funds from ATMs during the day.
Tap water is potable, but bottled water is widely available, albeit pricey. Always carry water when hiking. Having travel insurance is a must as everyone must pay to visit a general practitioner, even UK citizens. UK citizens can receive free emergency treatment, along with EU citizens holding an EHIC card.
The roads on Guernsey are easy, but get busier in July and August. North American visitors will find country roads especially narrow and winding and the maximum speed limit is a sedate 35 mph. The beaches are safe as many reside in arching bays and have shallow sands. Keep kids covered up during the heat of summer as the sun can be intense in Guernsey.