I have a cd player on my boat with 4 channels rated at 50W each but i want to add 4 more speakers do i have to get a 8 channel amp or can i wire the speakers in series or parellel somehow?
I wouldn't suggest you wire any more speakers from the internal amplifier. This is because it won't provide sufficient power for both speakers if you wire it in series, and if you wire it in parallel then you're risking ruining the amp because headunits aren't two ohm stable.
The best way to do this would be simply buying a four channel amplifier and adding the new speakers to that or if you could find one that was two ohm stable with a more powerful output, then you could start wiring two speakers to each output.