The　growth　of　maritime　activities　has　boosted　the　demand　for　efficient　marine　communications　and　computation　offloading.　Mobile　edge　computing　(MEC)-powered　space-air-ground-sea　integrated　network　(SAGSIN)　is　a　promising　solution　to　satisfy　these　demands.　To　the　best　of　our　knowledge,　the　existing　research　on　high　altitude　platforms　(HAPs)　in　SAGSIN　remains　open.　To　exploit　the　HAPs＇　advantages　of　facilitating　communication　at　shorter　distances　and　more　stable　computing　services　than　terrestrial　base　stations,　this　work　considers　multi-HAP-assisted　computation　offloading　in　SAGSIN　and　formulates　an　optimization　problem,　which　turns　out　to　be　a　mixed　integer　nonlinear　programming　(MINLP)　due　to　the　joint　optimization　among　task-HAP　association,　computational　resource　allocation　of　HAPs　and　task-satellite　association.　To　this　end,　the　problem　is　decoupled　into　three　single-variable　subproblems　by　relaxing　the　delay　constraint,　and　the　subproblems　are　solved　by　graph　theory,　the　Lagrange　multiplier　method　with　Karush-Kuhn-Tucker　(KKT)　constraints,　and　the　alternating　optimization,　respectively.　Simulation　results　demonstrate　the　efficiency　of　the　proposed　scheme　with　superior　performances　compared　to　benchmark　schemes.