Unfortunately mildew is a common problem on boats. Commercial mildew removers are expensive and have a limited life in storage. Apart from the obvious of keeping dry and ventilation, here are a couple of tips for combating mildew.
MILDEW REMOVALYou will need to purchase some concentrate. I use AQUA CHEM spa care SHOCK TREATMENT. It is a white powder and the active ingredient is Lithium Hypochlorite, 29%. It is sold as a chlorinating shock treatment for spas and hot tubs and you will find it, or an equivalent, at any pool chemical supply. I purchased 16 ounces for $5.97 at Walmart. 16 ounces should supply enough mildew remover for approximately 50 years on a 71 foot boat!
Mix in the ratio of one level teaspoon of powder to one gallon of water. That teaspoon full doesn't look like it will be enough in that big gallon of water but don't be tempted to put more in - the powder is super concentrated and this ratio works very well. Since the label says it is a Federal offense to use this product other than indicated on the label, I suggest you take your boat out beyond the three mile limit before performing these illegal acts.
The solution should be kept in a sealed container, plastic bottles with a screw cap work well. Label well to avoid accidents. Transfer as required to a spray bottle but don't store it in the spray bottle since the air vent seems to allow the strength to weaken in a few days. Even in the sealed container it lasts only a few weeks but it is so cheap compared to commercial preparations - go ahead, make another gallon.
Don't dispose of excess solution or powder in the water unless extremely diluted as it is toxic to marine life. If left over solution gets too weak to use, I dilute it in my bilge and allow it to sit there exposed to the atmosphere for a few days before pumping overboard. So long as there is no oil in your bilge this helps keep your bilge clean and kills any smelly bacteria before emptying. I have a large bilge and plenty of ventilation, smaller boats might need to find an alternate disposal method but in small quantities it should not be worse than having an indoor spa.
On smooth surfaces - paint, vinyl, varnish etc., - spray on, leave a few seconds to work, and wipe off. You can even wet a paper towel and just wipe slowly and it will work. There is no need to rinse the surface, in fact I think the residue inhibits the re-growth of mildew. There is some chlorine smell so make sure you have plenty of ventilation.
REMOVING MILDEW FROM DOCK LINES, CANVAS, RIGGING AND ROUGH SURFACESChemicals and scrubbing work fine on smooth surfaces but on halyards, dock lines, awnings, etc it is impossible to scrub down into all the cavities so a black residue is left which not only still looks dirty but is the seed for new algae to start growing.
The easiest way to get these looking like new is a pressure washer. You can do lines without removing or lay them down and move them a couple of times until you get all sides. It doesn't take much pressure and you may do some damage with a commercial high pressure unit so experiment with a pressure adjustment if there is one, or back off sufficiently so there is just enough pressure to remove the dirt and mildew. Just use plain water, you don't need to add any soap, detergent chlorine or Lithium Hypochlorite.
MILDEW PREVENTIONMost marine stores and catalogs sell a product called DI-GAS or MIL-DU-GAS and I have occasionally seen the equivalent in WalMart. These products are labeled "HAZARDOUS TO HUMANS AND DOMESTIC ANIMALS" and contain PARAFORMALDEHYDE. When not sold to the marine market these products are labeled "NOT FOR INDOOR USE". With the restricted ventilation in boats this warning should be more applicable! CAUTION, they are quite nasty, in fact, if you use it as instructed you boat will become uninhabitable. DO NOT USE AS INSTRUCTED.
Despite the above warnings against these products, they do work well to prevent mildew and the "boat odors" in closets etc. Use these products by opening the seal as instructed but then enclosing them in a zip lock plastic bag. Cut a ¼" hole high in the side of the bag and hang in your closet. The reduced dissipation is still sufficient to prevent mildew and the bag of poison lasts a year or more instead of a couple of months. This works well for enclosed spaces but I do not recommend trying to control mildew by treating the whole boat space, unless the boat is unoccupied for an extended time.