Vessel for performing subsea wellbore related activities, wherein the vessel comprises: - a floating hull having a main deck with a deck surface and a moonpool with a firing line extending there through, - a hoisting device, e.g. a crane or multipurpose tower, which hoisting device is mounted on the floating hull and is configured for lowering and lifting an object along the firing line and via the moonpool towards and from a sea floor respectively, said hoisting device comprising: ∘ a hoisting winch and hoist cable connected to said hoisting winch, wherein the hoisting device is configured to support the hoist cable along the firing line; ∘ a travelling block supported by the hoist cable for connecting with the object; and ∘ a heave compensation system adapted to provide heave compensation for the hoist cable to provide a heave compensated motion of the travelling block; - a vertically mobile working deck, the mobile working deck having a working deck surface, which mobile working deck in a lowered position thereof covers the moonpool, and which working deck surface of the mobile working deck is level, preferably flush, with the deck surface of the main deck of the hull; - a mobile deck compensator, which mobile deck compensator is connected between the floating hull of the vessel and the mobile working deck, wherein the mobile 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, 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, characterized in that the mobile deck compensator comprises in series: - a lift cylinder, configured to lift the mobile working deck out of the lowered position and to move the mobile working deck between the lowered position and an elevated position, and - a heave compensation cylinder, configured 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. - a floating hull having a main deck with a deck surface and a moonpool with a firing line extending there through, - a hoisting device, e.g. a crane or multipurpose tower, which hoisting device is mounted on the floating hull and is configured for lowering and lifting an object along the firing line and via the moonpool towards and from a sea floor respectively, said hoisting device comprising: ∘ a hoisting winch and hoist cable connected to said hoisting winch, wherein the hoisting device is configured to support the hoist cable along the firing line; ∘ a travelling block supported by the hoist cable for connecting with the object; and ∘ a heave compensation system adapted to provide heave compensation for the hoist cable to provide a heave compensated motion of the travelling block; ∘ a hoisting winch and hoist cable connected to said hoisting winch, wherein the hoisting device is configured to support the hoist cable along the firing line; ∘ a travelling block supported by the hoist cable for connecting with the object; and ∘ a heave compensation system adapted to provide heave compensation for the hoist cable to provide a heave compensated motion of the travelling block; - a vertically mobile working deck, the mobile working deck having a working deck surface, which mobile working deck in a lowered position thereof covers the moonpool, and which working deck surface of the mobile working deck is level, preferably flush, with the deck surface of the main deck of the hull; - a mobile deck compensator, which mobile deck compensator is connected between the floating hull of the vessel and the mobile working deck, wherein the mobile 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, 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, - a lift cylinder, configured to lift the mobile working deck out of the lowered position and to move the mobile working deck between the lowered position and an elevated position, and - a heave compensation cylinder, configured 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.