I definitely agree on getting it setup to surf first. I'm glad you have your priorities in order
Tunes are good and all, but riding is definitely where it's at.
In regards to the amp, it can run everything, but looking at the specs of your amp and just the 4 tower speakers, you would be running them close to what they want to run, splitting it off further to another 4 in boats is underpowering them and not running them at their potential.
Let's take a look at your amp first:
Main Channels @ 4 Ω 75 W RMS x 4
Main Channels @ 2 Ω 100 W RMS x 4
Main Channels - Bridged @ 4Ω 200 W RMS x 2
Subwoofer Channel @ 4 Ω 180 W RMS x 1
Subwoofer Channel @ 3 Ω 240 W RMS x 1
Subwoofer Channel @ 2 Ω 300 W RMS x 1
and now your tower speakers:
Continuous Power Handling (RMS) 100 W
Peak Music Power 225 W
Recommended RMS Amplifier Power (per Ch.) 40 - 175 W
System Efficiency 91.0 dB @ 1 W / 1 m
System Nominal Impedance 4 Ω
System Frequency Response 45 Hz - 25 KHz ± 3 dB
Having 1 speaker per channel, you are getting 75W RMS to each speaker
This is enough to power them, as they call for 40-175. Typically you will want to run at their continuous RMS of 100W. You are just short of this, but are in the recommended range for them to work. As a rule of thumb, you want the amp to be more powerful than the speakers and turn the amp down some so it is not trying to work so hard.
Now let's add in your 4 in boat speakers. Let's say you wired your speakers in parallel to double the amount of speakers and drop your impedance to 2 ohms. With this you would have 1 pair of speakers wired to each channel.
Your amp would then deliver 100W RMS, or 50W RMS to each speaker. It is still within the recommended range, but half of what you should be aiming for.
I don't know which JL 6.5" speakers you have for the in boat speakers, but they would also be receiving 50W RMS each.
Now as JL said, yes it can run them. Is it optimal or the best setup you can run everything on? No. It will work and get you by, but you are not utilizing your speakers as they should be. As it sits you are running everything at half power and can get a lot more out of your system then it is giving you now.
Leaving this amp for your in boat speakers and getting say a M400/4 (the same amp you have now without the sub channel) if you wanted to stick to a strictly JL setup, to power the tower speakers would be recommended. If you wanted to switch to another brand of amp, I would get something that was equivalent in power or maybe even something a little more beefy putting out 100-150W RMS per channel would be the best choice to maximize your current speaker setup.