Alfred the Great built a navy and used a system of fortified towns, called burhs, to help fight off Danish raids. I wrote about this before; the pertinent section is probably this one:

In terms of how Alfred was able to successfully defend against the Norse, fortified towns (burhs) were fortified locations, usually linked to settlements, linked by roads, that could command key fords or crossings; sometimes double-burhs linked by a bridge would bar passage along or across a river. Alfred teamed the burh system with a mobile army, mostly cavalry, that could quickly respond to attacks. It was similar to the system that Charles the Bald was developing in the Frankish empire, and one that Offa of Mercia had developed about a century before Alfred (Offa, though, lacked Alfred's purpose-built fleet).

The cavalry could alert nearby ships leading to small raids and ambushes that would occur when men had left their ships and their retreat was cut off by an arriving force. The Anglo-Saxon Chronicle has a description of such an action in 896, where a force of six Danish ships was raiding along the Isle of Wight. Alfred sent a force of nine ships to intercept them; when they found the Danes, three ships were beached with a small shipguard while the others were upriver raiding. The English ships attacked the guard ships and captured two of them, while one escaped, but in the process the English ships were grounded on an ebb tide, with only three of the ships on the same side as the rest of the Danish fleet and the other six on the other side. The Danes returned from raiding and attacked the three English ships, losing 120 men to 62 English killed. The battle ended when the rising tide re-floated the Danish ships before the bigger English ships, but only one escaped.