We regularly run 6 in-boats and a sub on a 5 or 6 chnl amp, so for 4 in-boats, sub and a pair on the tower, I see no issue with the M700/5 handling this load.
But, for the tower speakers, especially a single pair of coaxial, I would suggest a bit more power then 75W rms. So I would go with a dedicated amp for the tower speaker. This will also be a slight benefit for the 4 in-boats. With each on their own chnl, they will receive 75W rms as opposed to 50W.