Word of mouth has always been the best and strongest form of advertising and word of mouth advertising comes from doing top notch work that out shines the competition.
So as basic as it can seem start by what I'm sure you're already doing and that's top notch work.
Next if you don't have a Facebook page for your business start one. It's fast and easy and best of all it doesn't take any special software, knowledge or skill. (unlike creating a traditional website).
Then started feeding your Facebook page with your work. The easiest way to do this is with a quality camera smart phone. I use an iPhone but other phones will work just as well.
Take good before and after pictures and then simply include a few words describing the work performed.
Your contact information should be available when people click the "about" text link on your Facebook page.
Keep it clean and simple. Include your names, business name and most important your contact information.
When it comes to including your phone number, keep it simple but complete. This means include your area code - prefix - number.
Like this,
000-000-000
Avoid doing things like this,
800-boatwax
As it's so annoying to work at typing in a number via an alphabet.
Also, after completing a job for a customer, be very proactive in asking for referrals. Tell your customers something honest and simple,
Hey John,
My wife Julie and I are always looking for more good customers like yourself. We take pride in our work and always treat our customer's boats as though they are our own. Here's a few of our business cards if you have friends looking to have someone to detail and take care of their boat.
Our Facebook page, phone number and e-mail are all on the car.
Keep it low pressure but do as for referrals and DO include your Facebook page address on your cards as it's the easiest and fastest way for a potential customer to see before and after pictures of your work.
This circles back to starting a Facebook page and then being proactive on feeding it.
That's off the top of my head for now....