This photo shows a few of the different types of crimp terminals you can purchase. The terminals in the top row are marine grade crimp connectors with built in adhesive lined heat shrink and are the best possible solution, for most applications, to use on a boat.
As I mentioned there is no such a thing as marine grade terminals but that is what the better quality terminals have become known as. Heat shrink terminals, as seen in the top row, are expensive, but in my opinion almost always well worth it. They are available from manufacturers such as Ancor Products, AMP, and FTZ.
NOTE:When choosing crimp terminals I would urge you to buy from companies with a reputation to uphold and a long history for quality. If you buy from companies such as AMP/Tyco, Molex, FTZ, Burndy, Thomas & Betts (T&B), 3M, Ancor etc. and you will get better quality terminals than the cheap no-name junk that has infiltrated the country over the last 20 years or so.
The second row of connectors are called insulated terminals but are not heat shrinkable. The ring terminals are a what are considered a three-piece terminal and the butt splices are a solid tube with no seam. These terminals stand head and shoulders above the bottom row but keep in mind that they are not sealed connections. As such they are quasi-open to the marine environment. When used in conjunction with UL 1426 tinned marine grade wire these crimp terminals will last a long, long time provided they are not in any direct contact with water, such as in a bilge.
The third row represents Wal*Mart or Harbor Freight quality terminals that should be avoided on a boat. Heck I'd not personally use this crap in a child's tree-fort let alone a boat. Perhaps the biggest issue is that, being of the bottom feeder variety, they tend to vary tremendously in ID and OD and as such it is a crap shoot as to how a crimp will turn out.
A good tip when buying connectors is this; if you can't see through the insulation, it's most likely vinyl, and should really be avoided. The vinyl insulation on this type of connector can and will crack and the raw metal thickness, where the wire meets the connector to be crimped, is very, very weak in comparison to top quality terminals from reputable manufacturers such as those listed above.
TIP:I buy my FTZ heat shrink terminals from Sailboatowners.com (LINK)
For non heat shrink insulated terminals I use AMP PIDG Terminals and buy them where ever I can find the best price. Because they are AMP they are quite expensive so I search around......