Vessel for performing subsea wellbore related activities, wherein the vessel comprises: - a floating hull having a moonpool and a main deck with a deck surface and with a moonpool opening; - a hoisting device mounted on the floating hull and provided with a winch system, wherein the winch system is configured for lowering an object along a firing line through the moonpool to a seafloor, which winch system is configured for lifting an object from a seafloor along a firing line through the moonpool, said winch system comprising: - a hoisting winch and hoisting cable connected to said hoisting winch, which hoisting cable is suspended from the hoisting device along the firing line; - a travelling hoist block supported by the hoisting cable for connecting with the object to be lowered to, or lifted from, the seafloor along the firing line; and - a heave compensation system adapted to provide heave compensation for the hoisting cable to provide a heave compensated motion of the travelling hoist block; - a vertically mobile working deck, the mobile working deck having a working deck surface with a hoisting cable opening that is aligned with the firing line and that is configured for passing through at least the hoisting cable, which mobile working deck in a lowered position thereof covers the moonpool, and which working deck surface in the lowered position of the mobile working deck is level, preferably is flush, with the deck surface of the main deck of the hull; - a mobile deck compensator, which mobile deck compensator is connected with the floating hull of the vessel and with the vertically mobile working deck, wherein the mobile working deck compensator is configured to lift the mobile working deck out of the lowered position, to move the mobile working deck between the lowered position and an elevated position thereof, and to provide a heave compensated motion of the mobile working deck when lifted in the elevated position, moving the mobile deck between a heave compensation maximum height position and a heave compensation minimum height position; - an object loading device (45;24;81,82) configured for introducing the object below the mobile working deck and in the firing line, such that the object can be coupled with the travelling hoist block, to enable the winch system to lower the object along the firing line below the mobile working deck and to the seafloor. - a floating hull having a moonpool and a main deck with a deck surface and with a moonpool opening; - a hoisting device mounted on the floating hull and provided with a winch system, wherein the winch system is configured for lowering an object along a firing line through the moonpool to a seafloor, which winch system is configured for lifting an object from a seafloor along a firing line through the moonpool, said winch system comprising: - a hoisting winch and hoisting cable connected to said hoisting winch, which hoisting cable is suspended from the hoisting device along the firing line; - a travelling hoist block supported by the hoisting cable for connecting with the object to be lowered to, or lifted from, the seafloor along the firing line; and - a heave compensation system adapted to provide heave compensation for the hoisting cable to provide a heave compensated motion of the travelling hoist block; - a hoisting winch and hoisting cable connected to said hoisting winch, which hoisting cable is suspended from the hoisting device along the firing line; - a travelling hoist block supported by the hoisting cable for connecting with the object to be lowered to, or lifted from, the seafloor along the firing line; and - a heave compensation system adapted to provide heave compensation for the hoisting cable to provide a heave compensated motion of the travelling hoist block; - a vertically mobile working deck, the mobile working deck having a working deck surface with a hoisting cable opening that is aligned with the firing line and that is configured for passing through at least the hoisting cable, which mobile working deck in a lowered position thereof covers the moonpool, and which working deck surface in the lowered position of the mobile working deck is level, preferably is flush, with the deck surface of the main deck of the hull; - a mobile deck compensator, which mobile deck compensator is connected with the floating hull of the vessel and with the vertically mobile working deck, wherein the mobile working deck compensator is configured to lift the mobile working deck out of the lowered position, to move the mobile working deck between the lowered position and an elevated position thereof, and to provide a heave compensated motion of the mobile working deck when lifted in the elevated position, moving the mobile deck between a heave compensation maximum height position and a heave compensation minimum height position; - an object loading device (45;24;81,82) configured for introducing the object below the mobile working deck and in the firing line, such that the object can be coupled with the travelling hoist block, to enable the winch system to lower the object along the firing line below the mobile working deck and to the seafloor.