When crossing a large body of water, a rainstorm would be seen as an opportunity to wash off all the accumulated salt - the character would have undressed and lathered up immediately rather than just rolled up his sleeves. It may even be the first proper wash a sailor would have had in weeks, due to the limited supply of fresh water aboard.
When "Our Guy" (Redford) uses the radio, he tries to speak in the wrong side of the handheld microphone. He tries to talk into the metal latch, covering up the actual microphone opening with his hand. Any experienced seaman would never make this error.
When "our man" leaps aboard the inflatable life raft during the tempest, he neglects to untie the tether to the sinking sailboat and proceeds to ride out the storm before falling asleep. He awoke the next morning to find the boat still afloat and tether intact. The boat ends up protesting its own buoyancy minutes later after he decides to re-board and fetch more supplies, but could certainly have sunk hours sooner.
Although the part for "our man" was written to present the character as errant in certain scenes, no seasoned sailor would have made this mistake.
The second time the boat does a complete roll the hatch to the cabin is completely open, yet when the man returns to the cabin the floor is completely dry. Water would have poured in through the open hatch until forming an air bubble at the bottom of the hatch (then upside down).
Early on, "Our Man" is shown shaving with a manual razor and shaving cream. Over the next 8 days, much of that time fighting for his life in a small inflatable lifeboat, his face and chin remain smooth, with no signs of any stubble.
When then man puts on his storm suit and goes out to raise the storm jib he is tossed off deck and dragged underwater. He climbs back on board and returns to the cabin. When he removes the top of the storm suit his shirt is completely dry.
When Our Man deploys his sea anchor from the stern of the boat, the yacht should immediately swing round in the gale, presenting its stern to the wind, yet we see Our Man continues to get lashed with rain blown from the bow.
When Our Man releases the rope for the final time as he watches the Virginia Jean sink, his hands are inside the lifeboat. A second later there is a dramatic shot of us looking at him from the boat's position and he is leaning on the lifeboat with both forearms. A third shot, again looking at the sinking boat, shows his arms inside the raft. This whole sequence is very still, giving us no expectation that he would have rapidly moved between shots.
Our Man raises himself up the mast to reconnect a PL-259 RF connector to a small antenna. On his descent, the camera looks down. in some shots the deck is in full sun with the shadow of the mast cast on the deck. In others, there are no shadows.
It is not really correct to call "SOS" on the marine emergency radio channel. "Mayday" or "Pan Pan" would be accurate and standard forms whereas "SOS" will be understood but is incorrect. A veteran sailor would never have made this mistake.
When the boat is dismasted, "our man" uses a knife to cut the shrouds (i.e., the metal wires) that are still linking the stump of the mast to the section that is lying in the water. This is impossible, since only a bolt cutter with a long handle could do the job.
During the storm, "our man" picks up a bag with the "storm jib" label and then fights against the elements to hoist this sail. However, the sail in question is not a storm jib at all. It is very large, light and white, whereas storm jibs are small, heavy and brightly colored.
On the boat during the storm, "our man" ties the floating anchor to the aft (rear) pulpit. Given the huge forces involved, the pulpit would have been bent and ripped away in seconds. A similar comment could be applied when he ties his lifeline to the guardrails.
Only after his radio fails does he raise himself up the mast to reconnect a PL-259 RF connector to a small antenna that could not possibly transmit on anything but short distance "Marine Band" frequencies. The PL-259 is a knurled connector tightened with fingers or a pliers, but the man removes a Crescent wrench from his tool pouch and appears to be tightening the PL-259 and that's not possible.
The hardest items to dry - and therefore the ones most likely to be seen on the deck after the initial salt-water flooding - would be the sofa cushions and bunk mattresses. Yet apparently, these dried very soon after.
Day 1: Wading in thigh deep flooded cabin to find fiberglass hull repair kit
"Our Man" picks up a Panasonic "Fully Rugged" Toughbook laptop, examines it,
lays it open and flat to dry. Waterproof and washable (popular at sea) it'd likely be undamaged by flooding.
When our character drinks the sea water, after attempting his desalination technique, he checks his watch (seiko skx009) to see the time but it is actually worn in the wrong way.
The crown is in the wrong place therefore he would see the time upside down.
When the container is embedded in the side of the yacht at the start of the film, our man deploys a sea anchor to drag the container away. As the container and the sea anchor are being moved by the same ocean, the sea anchor would just sink and not exert any useful force on the container.
If he deployed a "sea anchor" which he used to remove the container from his boat, why did the rope remain relatively flat after the container was pulled from the boat? Wouldn't the rope be seriously pulling "straight down?"