If you want to read "The Seafarer" in Old English, get ready to put in some serious study time. Unless you're already an Anglo-Saxon scholar, you're going to need to learn the language first. That means memorizing scary-sounding things like noun declensions and verb conjugations – the whole nine yards.
And even after you do that, you'll find as you translate the poem that much remains mysterious and unclear to you. The language has been "dead" for so long that many word meanings and usages have been lost. Even the best Anglo-Saxon scholars don't always agree on what a particular phrase means. You'll be rewarded, though, by the beauty and complexity of the poem in its original language, which even the best translations can't completely capture.
But to have an easier go of it, you'll probably be reading "The Seafarer" in a modern translation, and there are some great ones out there. For example, the famous poet Ezra Pound wrote a beautiful, although not very accurate, translation. When reading any poem in a language other than its original, keep in mind that many literary features – fun things like word play, alliteration, meter, and the connotations of important words – have probably been lost in translation. If "The Seafarer" really floats your boat, you might want to put in the time to read a few different translations to develop a fuller understanding of the poem.