Amp, 6 spkrs and sub, yes or no?

I have a boat with a Clarion XMD3 head unit and 4 speakers currently.  I would like to install two more speakers and a sub, will I need an amp for this?  I believe I will.  If I do what is the best way to wire the system?  Will I need an amp that has 8 inputs? or can I put the speakers in series and use the other input for the sub?  I am new at this so any help would be appreciated, thanks.

Yes to the amp for sure. Describe further the type of speakers you have in the boat right now and where they are located. Some people incorrectly count component speakers as 4 speakers when they are really 1 pair of speakers as an example. I am still at fricken work, 4:45am and I have not made it home for the night yet....Top Secret, I can tell you but then my wife will kill me.