I HAVE 2 1990 MARK FOURS
WHAT WOULD BE THE BEST HYD ROLLER LIFTERS, CAM, & SPRINGS TO PUT IN?

Sorry, OPA.

You gave us a pretty wide range to work in here.
If your motors are Mercs, then they are anywhere from a 330 horse 454's to 500hp 540's. The cam choice will most certainly differ for those two motors.

Give us somethign to work with.
(and it's Mark IV, not Mark 4).

THEY WERE MERC 365S

Ok, you said they were 365's.
If they ARE 365's and you're not going to change anything but the cams, let them be 365's.

In other words, you can't expect much gain at all with out following this up with other modifications.

You have to tell us what you goal is. Top end hp, torque, fuel economy or something else.
And you have to specify your plans for heads, carbs, intake, pistons, exhaust,etc.

A cam change on a 365 is worth doing. His stock carb and intake are plenty big. If you go to a hyd roller you'll spend a pretty penny.

You are looking at a new timing set ($70), lifters ($400), cam ($280), valvesprings ($230), and gaskets.

If you're watching the $$, then go with a hydraulic flat tappet cam with lift around .550" and .050 duration numbers of around 225. You can get a decent cam kit with springs, lifters, and stick for around $350. Then you only need a timing set (and some kits come with that, too).

Blue Racer has a cam (WG1081K) that has a little more duration for $135 including lifters. If your boat is fairly light, then that cam may be your ticket. MrGasket has a $70 kit with timing set, all gaskets for the swap, and break in lube. Comp 924 Double springs will cost you $100. That's got you covered for under $350 per motor.

Comp has complete kits for around $450.

If the $$ aren't so much of an issue, then we can really get to talking...

The largest one you can afford cam intake and exhaust required if you go past 550 and 230 duration.

I HAVE HAD BAD LUCK WITH FLAT TAPPETS
HYDROLIC COST MORE BUT IM LOOKING FOR LESS PROBLEMS

No guarantee there will be fewer problems with rollers. Do a search on roller lifters. If you go roller cam, you will need new pushrods. Retrofit roller lifters are longer than flat tappets. You will have to fit a cam button to control cam end play. Roller rockers are also a requirement.

I would use a hyd. flat tappet cam like a Crane 132561. Specs are .530 lift intake, .551 exhaust, 228 duration @ .050 intake, 236 exhaust. Reversion is not a problem with stock exhaust. With the money you save you could buy a Holley 800 and a Wiend Accelerator intake. That would sort of give you a Merc. 420.