Temperatures may change enough between day and night to melt the surface of a gallium "sea" but unlike water, liquid gallium is opaque, so the stellar radiation can't penetrate and even slightly warm the liquid below the surface.
Therefore, if the sea is to become liquid at all, there needs to be enough heat flux from below (from inside the planet, that is) to keep most of the sea melted, so freezing only takes place at the surface (mainly due to radiative cooling at night). The result, given that solid gallium floats in the liquid much like ice in water, is that you'll get a skim of "ice" that slowly thickens the longer the surface stays cold.
During local winter, this "skim" might get thick enough to support substantial loads, as does ice on lakes or seas in subArctic and Arctic climates on Earth. In local summer, the gallium might remain liquid day and night, or barely skim over.
As noted above, most metals are bad choices for gallium sailing -- however, composites are not affected this way. Wood, composite or inflatable hulls might be a very viable choice, with sail power (engines always seem to want to put metal parts in the "water"). The hull(s) could be very small relative to the load, compared to what water-borne sailors are used to, leading to the likelihood of tricycle layouts with small floats supporting a load-bearing framework -- more like ice boats than Earth ships.
The brilliant part of this is that it would be almost impossible for a "man overboard" to drown -- hypothermia could be an issue, over time; despite being as temperate as a tropical Earth sea just to remain liquid, the gallium will conduct body heat away far faster than even sea water, but a person would float far higher than is the case even in hypersaline water like the Dead Sea (though not anything like as high as in old pictures of huge mercury pools). Standard overboard survival drill would be to lie still, on your back, with arms and legs spread for stability, and wait for rescue. Swimming would be more like crawling in very soupy mud than what we're used to.