The year 1853 was only four years after the Gold Rush in California. Though gold-seekers and other immigrants had caused a local population explosion, the infrastructure had not kept up. The San Francisco and San Jose Railroad was still ten years in the future, so those who wished to travel between those town had their choice between a bumpy and dusty stagecoach or horse ride down the nearly unimproved Spanish road (now El Camino Real), or taking passage on one of several shallow draft steamers plying the Bay.
Those who could afford it took the steamers. One such was the 61-ton side-wheeler 'Jenny Lind' - named for a famous Swedish singer of the time - running a route among the harbors at San Francisco, Redwood City, and Alviso (San Jose). On April 9th, 1853, the Jenny Lind was chartered for an excursion from San Francisco to Alviso. At two in the morning of the 11th, she departed the Alviso Embarcadero headed back to San Francisco, with a heavy load of 125 passengers, including 8 women and 7 children.
At half past noon of the 11th, the Jenny Lind was about half-way back to SF, off the 'Pulgas Ranch' (a Mexican land grant extending from current Menlo Park to San Mateo). The ladies, children and their male escorts had just been admitted to the after cabin to be seated for lunch. At that moment, the high pressure boiler of the Jenny exploded. Since the ship was shallow-draft, the boiler was on the same deck as the passenger cabin. The aft end of the boiler shattered, sending its fragments as well as shrapnel from an intervening bulkhead into the lunchtime crowd, followed by a searing, lung burning cloud of high pressure steam. All in the cabin was severely scalded, many having their clothes blown off and hair melted. The steam roared up a stairway to the upper deck, felling more. The forward part of the boiler took off like a rocket, obliterating the fireman who had been stoking it, and scalding or smashing into several others on the forward deck.
The Jenny was left adrift, with 10 killed or drowned immediately, 40 injured - most severely burned, and another 10 in the water having jumped or been blown overboard. It cannot have been far offshore, as passenger Peter Smith volunteered to swim to shore, from where word of the disaster was carried to San Francisco. Two steamers were sent out to carry the wounded and tow the Jenny back to the city, but it was ten hours after the explosion before the injured reached shore and assistance. In the meantime the burned victims, mostly beyond help by the medical practice of the day, had begun to die.
By the time the local correspondent of the New York Times filed a story, 31 had died. (The story was not printed until May 13th, due to the travel times of the era.) At least three more died later, making the total fatality count 34 or more, with at least 16 further injured. Due to the timing of the explosion, all of the children on board were killed, and at least six of the eight women. The dead included a former alcalde of San Jose, the owner of a 4,000 acre ranch, and the wife of a steamship line manager.
The exact location of the Jenny Lind tragedy is not known. A more recent account puts it 'not far north of the Dumbarton Bridge', and must have been close fairly close to shore, so this spot seems appropriate. You are searching for a small camo-taped plastic jar.