This photo shows a few of the different types of crimp terminals you can buy. The terminals in the top row are marine grade crimp connectors with built in adhesive lined heat shrink and are the best possible solution to use on a boat. Technically I don't believe there is such a thing as "marine grade" terminals but that is what the better quality stuff has become known as. Yes, heat shrink terminals are expensive, but worth it, and they are available from manufacturers such as Ancor Products, AMP, and FTZ.
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The second row of connectors are marine grade but non-heat shrink. While these terminals stand head and shoulders above the bottom row they are not sealed connections and as such are open to the marine environment. When used in conjunction with UL Listed tinned marine grade wire these crimp connectors will last a long time provided they are not in any direct contact with water such as in a bilge.
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The third row represents Wal*Mart quality connectors that should be avoided on a boat. They vary _tremendously in ID and OD diameters 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 be by-passed. 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.