There's some useful advice here:
http://www.wikihow.com/Write-a-Eulogy
My own instinct would be to 'keep it light' and, if possible, to inject a bit of humour. Only you can work out how that might be done (because only you knew what your Dad was like) but, just as an example, something like this might be appropriate:
"There are ordinary fishermen, who simply enjoy fishing; Dad was such a fisherman. And then there are dedicated fishermen, who venture onto the riverside in all weathers, when common sense clearly dictates against it; Dad was such a fisherman. There are also
true fishermen, whose catches weigh more every time they recount the battle to reel them in; Dad was also such a fisherman".