Certainly the Polk Audio dBs are a major player in the sub-$100 a pair coaxial category. But there is another level above that grade of speaker that makes a huge audible difference, especially when you add an external amplifier. Here is what you get with a Wetsounds or JL Audio marine coaxial as compared to most speaker products in the $100 range:
Rather than an interrupted midbass cone from a tweeter supporting pole piece you get a bridge/grill mounted tweeter and a continuous midbass cone. This results in more surface area and much better midbass performance. 50 percent of all musical fundamentals fall between 200 and 600 Hz and this is the sole domain of the midbass driver.
You might find a 'W'-dome tweeter in a less expensive coaxial with a much higher resonance and a more erratic response. A better speaker will transition to a true dome tweeter that has a much larger VC, lower resonance, is better damped and has a profoundly smoother response that impacts vocals, particularly female vocals.
From a stamped steel basket to a polymer basket.
Oftentimes going from a single pole highpass crossover to a two pole crossover. Steeper slope. More tweeter protection. less overlap. Smoother response.
A synthetic spider versus a linen spider. A linen spider will degrade in several seasons in a marine environment.
Your coaxials reproduce 7 of 8.5 octaves that are easily audible in a boat. This is where all the contrast and clarity comes from. These speakers determine the bass tonal construction and the bass pitch accuracy.
It escapes me why many choose this as the area to compromise. To my ears these are the most important audio components in the boat worthy of the largest investment.