how necessary is an amp/ and or sub

I have a pioneer HU I am wanting to put in the parents boat. We will likely only have this boat 2 more summers so we're not wanting to go all out. Would it be fine to just hook up 4 - 6.5's without an amp or is an amp really necessary to run it high pass filter and then use a sub for the bass ?

The choice is yours. Do you want high quality sound or not. You can get by fine with running the speakers off the deck. But if you want quality sound then you need amps and a sub. Just remember, whatever you put in the boat you can take out before you sell. Well as long as you don't start cutting into the boat.

You don't have to have a sub. But an external amp on the 4 speakers in the boat will definitely improve the sound quality. Especially, at higher listening volumes. The stock amp in the Pioneer head units only put out about 18 watts per channel and it distorts very quickly at higher volumes.

You won't have much bass output but the sound will be a lot cleaner.

What is the wattage of your head unit. If you have one thats like 40x4 or 50x4, with 6.5's, you shouldn't need an amp. your head unit should push enough to give off really good sound. Subs do sound better with an amp, even if it's a 100-300 watter. You can always remove the amp and sub when your ready to get rid of the boat. check your head unit out first and if your head unit doesn't have rca output jacks(red/white connections on the back), you might consider getting a high powered head unit or you can get an adjustable line level converter(this allows you to hook up an amp to a factory head unit).
hope this helps- good luck

Robert, he listed that he has a Pioneer HU. Pioneer HU's are rated at 50x4 maximum wattage but have only 18w rms at 1% distortion.

An external amp will definitely improve sound quality.