An interesting area of inquiry which I don't think has been raised here until now. A surviving report by an agent of the US Immigration Commission, travelling incognito in 3rd Class on one of White Star's 'big four' just a few years before the debut of the Olympic Class, records that the 'Hebrew' passengers were looked after by a Hebrew crewman who was employed as a kosher cook but also 'appointed by Rabbi as guardian'. This man claimed that he was the first to be engaged in such capacity and had pioneered this work on most ships of the line. His duty was to ensure that Jewish passengers were not provided with sub-standard accommodation and that their kosher food was provided and properly prepared (by himself). Though the Jewish passengers in 3rd Class tended towards a self-imposed segregation, a group of single women suffering badly from mal-de-mer complained that they received little attention from the stewardess-matron, and some considered that there was in general 'a strong anti-semitic spirit among the crew'.
In the Edwardian period it's likely that the 'guardian' would have arranged services, perhaps in the 3rd Class dining room. At that time, of course, these could not have been open to all Classes. In the mid-1930s, when liners began to provide synagogues as a permanent facility, these were generally located in 3rd Class but open to passengers from all Classes. The synagogue in the Queen Mary, specially trimmed with wood from Palestine, was dedicated by Britain's Chief Rabbi and was seen at the time as a welcoming gesture to Jewish emigrants, who were becoming increasingly unwelcome on German ships.
There is plenty of information on other sites. As can be seen, the full traditional observance is quite elaborate and requires much early preparation, as no work may be done during Sabbat.
I contacted a member of our local Jewish congregation and asked what devout Jews would have done on board Titanic, where full observance would have been difficult, though it might be noted that kosher food was available. White Star looked after its Jewish passengers.
I was told that much depended on how devout the Jews concerned were. Some may have not observed Sabbat at all. I myself noticed that during the Sabbat of 29 June 1912, Sir Rufus Isaacs, an eminent Jew, was at work in Lord Mersey's court.
Those more devout would have done their best by observing tradition as much as was feasible. This is considered quite acceptable, as it is believed that it is what is in the heart that matters.
Being a Catholic-Christian with ancestry from Poland (which has a Jewish culture) I don't recall any services, but in some books I have read like the other members on this forum, there was a chef in Steerage that cooked Kosher styled.
If there were enough men to form what is called a 'minyun' (lit., “count” or “number”) they would gather together somewhere convenient to pray as a group. Otherwise, they would do so individually if so inclined. The minimum to establish a minyon is 10 men of 13 years of age or older.