Is an amp for tower speakers completely necessary?
 

Our local dealer said that he recommends one, but is it completely necessary? My brother and I would like to get some tower speakers for the new boat our family just got, but don't want to buy an amp if we don't 100% need one. We wouldn't be getting anything that powerful, maybe something like a pair of single barrled wetsounds xst-650's. They will be going on a wakesetter illusion x tower. They don't have to be ridiculously loud- they will be used mainly for wakesurfing. So....do we need an amp?

Yes, you will definitely need an amp! My boat came with a Kicker ZX200.2 powering pro 60's and you could just barely hear them when surfing so we moved up to the IX500.4 and now run 250w to each speaker and now we can hear them. I would say you need at least 150w rms to hear them unless you are running a fresh air exhaust system on your boat.

yes, it's absolutely necessary.

Yep. You will need an amp for those tower speakers.

%100. Yes.

other than an amp, how is your dealer proposing to power/drive these new tower speakers? Was he suggestion that they can be wired to an existing amp? If so, then a "new" amp may not be needed as stated. If he was suggesting to wire them to the head-unit, then I would suggest an amp. You would be extremely disappointed in any tower speaker driven with only head-unit power. You do need some solid power for a tower speaker in order to overcome the wind, water and engine noise ans 18W rms from a head-unit will just not get the job done.

Any speaker that is elevated up on the tower in open air, instead of down in the bathtub (cockpit), will automatically require far more power. Then add the fact that you are driving the tower speakers MUCH harder to project over a longer distance, rather than 5 feet away as you would with coaming speakers, and listening directly in the face of exhaust noise.
You need 75% minimum, up to the full thermal rating of the particular speaker for the tower. Since you're running the tower speakers in the highpass mode, I wouldn't hestitate to go a bit over the speaker's RMS power rating. In this case, with the XS-650, you are talking 75 min. to 100 watts per EACH tower speaker (not per set).
Ignore the false power ratings offered by source units. They're not even close to reality. Only look at honestly rated external amplifiers. Usually when you add an amplifier, you must also add supply cable and maybe distribution items.

Would you also suggest an equalizer so I can adjust the volume of the tower speakers compared to the boat speakers?

Maybe. For an EQ to be a fully functional zone control it requires that all speakers are driven by external amplifiers rather than a portion by HU power.
You can accomplish this by also using the HU preamp fader as a zone control (front/in-boat, rear/tower). If the location of the HU fader is too inconvenient or too slow (buried in a menu), then an EQ or dedicated preamp zone controllers/line amps (Kicker, JL Audio) will do the job. The line amp controller provides this function at the driver's fingertips without long to and from RCA runs since only a non-audio path control cable makes the transverse run.