For boat coastal cruising in New England summers, out of water in winters.  Engine maintenanced based on roughly 150 hours per year, diesel, with fresh water cooling.

A boat in use around the year, especially in warm tropical or semi-tropical water, will have substantially compressed maintenance cycles.

This guide is very crude, and I don't follow it rigidly.  It primarily reminds you that everything on a boat needs maintenance, some things more often than others.

almost daily
check engine oil
check coolant
check seawater intake filter if motoring in a lot of grass

50-100 engine hours
change oil
check battery fluid
check transmission oil

1 year cycle
varnish exterior, mast
bottom paint
wax topsides
polish metal
zincs (rudder, propeller shaft, heat exchanger)

drain water from bottom of fuel tank
drain replace gallon of engine anti-freeze and replace 
engine fuel filters
new water pump impeller
clean bilge pump filter

flush bilge 
check automatic bilge switches
flush water tank, maybe clean
grease seacocks
check hoses, hose clamps
clean vacuum reliefs on outlet lines (head, bilge pumps)
grease steering cables (check for wire deterioration)

grease steering bearings
rinse sails, minor repairs to sails, sail covers
dinghy outboard: spark plugs, fuel filter

2 year cycle
engine belts
dinghy outboard: water pump impeller
grease winches
drain/change transmission oil

5  year cycles
rebuild head
replace head outlet lines
clean lectra san
pack stuffing box
cutlass bearing
check engine alignment grease windless thorough inspection of working sails
Interior 
varnish interior, paint

10 year cycles
rebuild plastic hatches
check chain plates, chain plate bolts,
bobstay fitting, tangs
refrigerator hoses
electronics (after 10 years, substantially obsolete and unserviceable)
(Maybe) rebed deck hardware, hatches
paint interior lockers 
have LPG tanks re-certified

20 year cycles
propellershaft?
pull out and check rudder shaft for corrosion when propeller shaft is out 
exhaust system?
new sails?
replace running rigging (unless chaffed earlier)
re pack rudder stuffing box
galvanize chain, anchor
plumbing hoses, copper supply tube

30 year cycles
rebuild deck
replace standing rigging 
through hull seacock fittings
(more often if boat is moored at a dock and subject to electrolysis)
engine re build or powering
replace mast wiring
rudder pintal /gudgeon bushing 
steering wheel shaft bushings

Check regularly and fix when needed:
standing rigging (especially swaged terminal fittings)
running rigging
hull blisters 
steering cable
rudders 
electrical components, wires, batteries, etc.
tanks are a maintenance item, but it is hard to generalize, replacement cycle depends on material and installation
lpg gas lines
leaking portlights
leaking deck fittings
bulkhead tabbing
mast glue joints 
keel bolts (check when convenient, when tanks are removed)
rebuild/replace bilge and fresh water pumps
hoses, hose clamps, fuel lines