The wire in the tower should be fine for a pair of speakers. What ones are you thinking of? The bad boys are the Rev10s but there are also other choices.
You definitely want to get another amp for it to power them. Depending on what speaker you go for, you want to get an amp that puts out at least the same, if not more RMS than the speakers' RMS rating to get the most out of them. No one likes an underpowered speaker that starts clipping at high volumes.
Get the Wetsounds WS420 (or it's variants, SQ or BT) to be able to control the zones and tune the channels. It only needs one pair of RCAs from the head unit and it controls the signal going to power your in boats, towers, and sub. It is an awesome EQ and makes everything work so much nicer.
It is very doable as a DIY, especially if you already have an amp installed.
If you have big enough wire going to your existing amp you can get a distribution block/y adapter for it and split off between the two amps, you can also do this with the remote wire. Make sure it has it's own ground though.
You can possibly steal a pair of RCAs you have going to your amp since the wetsounds just uses 1 pair of RCAs for each zone, not 2 for front/rear. You would just use 2 y adapters for the front/rear of your 5 channel (i'm assuming it's a 5 channel) your amp also may have an option to internally use both.
To install the wetsounds you can splice off the power wires going to your head unit since you are going to be mounting it somewhat close anyway, if not right next to it.
Aside from that, if it's prewired and you already have speaker leads where your amp is, then mount the speakers with tower clamps, angled up a little bit so they are poined out behind the boat when factoring in some bow rise.
Hooking up all the components isn't too hard in itself, it just takes a little time. It is totally doable by yourself. The main thing is take your time and pay attention to where everything goes.
I know a lot of people here on the forums, myself included will be happy to help you out with any advice if you get stuck.