/u/dontdreddonme 's answer is actually succinctly correct, as far as we can tell. Our only real source to talk about Viking maritime activity against other nations, as opposed to riverine manouevre or simple transport, is the Anglo-Saxon Chronicle, which records to great extent the campaigns of the English against the Vikings. The propagandistic nature of the ASC is not to be overlooked, and of course one might assume that as a source it will try and downplay or simply omit English defeats, but given that the Chronicle does record English defeats on land, we have to assume it's equally honest as to maritime events. The ASC records a number of fleet actions against the Vikings by the English, some of which are led by King Alfred in person, and it seems to be the case that the English fleet is a highly effective means of warding off Viking attack. Indeed on many occasions, Viking fleets are routed or their ships are captured by the English.

The fleet was definitely an important element of English defence, and one provided for extensively in land charters and in wills, many of which demand service for a vigilis marinis or for the equipment of ships or weapons and armour for sailors.